The following correction was printed in the Guardian's Corrections and clarifications column, Saturday 30 January 2010

Tove Jansson, part 2: As well as ­describing the Finnish author and artist Tove Jansson as Swedish, this review said that her novel The True Deceiver was set in a Swedish village. This ­error appears in a plot summary on the book's back cover; the text itself does not specify a country, and the point is reinforced in a disclaimer

After the enduring international success of her Moomintroll fantasies, the Finnish author-artist Tove Jansson, in her 60s, began to write adult fiction. It has taken a while for these books to get much attention outside Scandinavia. On the patronising assumption that books for children are nice, ie morally bland and stylistically infantile, critics, reviewers and prize juries often dismiss those who write them as incapable of writing seriously for adults  a prejudice which, transferred to painting, plays a part in the plot of The True Deceiver.

Anyone familiar with Jansson knows it would be unwise to dismiss her or patronise her work on any grounds. Her books for children are complex, subtle, psychologically tricky, funny and unnerving; their morality, though never compromised, is never simple. Thus her transition to adult fiction involved no great change. Her everyday Swedes are quite as strange as trolls, and her Swedish village in winter is as beautiful and dangerous as any forest of fantasy.

If a transformation has taken place, it is in the nature of her writing. The language is more than ever spare, lean, taut, minimalist. These adjectives describe a good deal of modern narrative prose  the modishly anorectic style, well suited to thrillers, police procedurals and the existential noir, but very limited in range. Jansson's range, though effortlessly controlled, is great. Her spare exactness can express not only tension and stress but deeply felt emotion, expansion, relaxation and peace. Her description is unhurried, accurate and vivid, an artist's vision. Her style is not at all "poetic"  quite the contrary. It is prose of the very highest order; it is pure prose. Through its quiet clarity we see unreachable depths, threatening darkness, promised treasures. The sentences are beautiful in structure, movement and cadence. They have inevitable rightness. And this is a translation! Thomas Teal deserves to have his name on the title page with Jansson's: he has worked the true translator's miracle.

I wish I could quote whole pages, but a paragraph must do:

If it got really cold, it didn't make sense to go on working. The shed wasn't insulated, and the stove was barely able to warm it enough to keep their hands from stiffening. They locked it up and went home. But on the seaward side where the boats were launched, the doors had a latch that was easy to open. Mats would go out on the ice with his cod hook and when no one was in sight he'd go into the boat shed. Sometimes he'd go on with his work, usually details so trivial that no one noticed they'd been done. But most times he just sat quietly in the peaceful snowlight. He never felt cold.

The main characters are Anna Aemalin, a successful illustrator of children's books, and Katri, whose only love and ambition is for the younger brother left in her care, Mats, a shy, slow, gentle fellow. Then there are honest Liljeberg the boat-builder, the wise Madame Nygard, the malicious storekeeper, a little horde of village children, and Katri's dog. Nameless, silent and yellow-eyed, the dog is yellow-eyed Katri's creature. And she flatters herself on her own wolfish superiority to other people: "My dog and I despise them. We're hidden in our own secret life, concealed in our innermost wildness."

No one in the village seems to be married, and the relationship that will form between the two solitary women, Katri and Anna, is not sexual, though it is intensely passionate, fiercely unstable, destructive and transformative. Anna, far wealthier than Katri, keeps her parents' house piously unchanged, and illustrates little books for which the publisher provides the words. Her paintings are marvellously truthful depictions of the forest floor, patterns of leaf, twig, moss, lichen . . . to which she adds the cute bunnies of the publisher's texts. She spends much time answering letters from her child readers, and none in looking after her business interests. She sleeps, sleeps all winter until spring comes and she can see the living ground and paint it.

Wolfish young Katri, determined to provide security for her brother, and also the fishing boat that is his one heart's desire, fakes a robbery of Anna's house in order to make her afraid to live alone, and pushes her way into Anna's service and confidence. Before long she appears to be in full control and has thrown out all the old furniture and the comfortable lies that let Anna sleep. But Anna, awake now, is not the bunny-rabbit she seemed, any more than Katri is truly the wolf. The unfolding of their story through vivid contrast and interplay of truthfulness and deceit, purity and complexity, ice and thaw, winter and spring, makes the most beautiful and satisfying novel I have read this year.

 This article was amended on Monday 14 December 2009. The original referred to Tove Jansson as Swedish. This has been corrected.

The following correction was printed in the Guardian's Corrections and clarifications column, Saturday 30 January 2010

Tove Jansson, part 2: As well as ­describing the Finnish author and artist Tove Jansson as Swedish, this review said that her novel The True Deceiver was set in a Swedish village. This ­error appears in a plot summary on the book's back cover; the text itself does not specify a country, and the point is reinforced in a disclaimer

After the enduring international success of her Moomintroll fantasies, the Finnish author-artist Tove Jansson, in her 60s, began to write adult fiction. It has taken a while for these books to get much attention outside Scandinavia. On the patronising assumption that books for children are nice, ie morally bland and stylistically infantile, critics, reviewers and prize juries often dismiss those who write them as incapable of writing seriously for adults  a prejudice which, transferred to painting, plays a part in the plot of The True Deceiver.

Anyone familiar with Jansson knows it would be unwise to dismiss her or patronise her work on any grounds. Her books for children are complex, subtle, psychologically tricky, funny and unnerving; their morality, though never compromised, is never simple. Thus her transition to adult fiction involved no great change. Her everyday Swedes are quite as strange as trolls, and her Swedish village in winter is as beautiful and dangerous as any forest of fantasy.

If a transformation has taken place, it is in the nature of her writing. The language is more than ever spare, lean, taut, minimalist. These adjectives describe a good deal of modern narrative prose  the modishly anorectic style, well suited to thrillers, police procedurals and the existential noir, but very limited in range. Jansson's range, though effortlessly controlled, is great. Her spare exactness can express not only tension and stress but deeply felt emotion, expansion, relaxation and peace. Her description is unhurried, accurate and vivid, an artist's vision. Her style is not at all "poetic"  quite the contrary. It is prose of the very highest order; it is pure prose. Through its quiet clarity we see unreachable depths, threatening darkness, promised treasures. The sentences are beautiful in structure, movement and cadence. They have inevitable rightness. And this is a translation! Thomas Teal deserves to have his name on the title page with Jansson's: he has worked the true translator's miracle.

I wish I could quote whole pages, but a paragraph must do:

If it got really cold, it didn't make sense to go on working. The shed wasn't insulated, and the stove was barely able to warm it enough to keep their hands from stiffening. They locked it up and went home. But on the seaward side where the boats were launched, the doors had a latch that was easy to open. Mats would go out on the ice with his cod hook and when no one was in sight he'd go into the boat shed. Sometimes he'd go on with his work, usually details so trivial that no one noticed they'd been done. But most times he just sat quietly in the peaceful snowlight. He never felt cold.

The main characters are Anna Aemalin, a successful illustrator of children's books, and Katri, whose only love and ambition is for the younger brother left in her care, Mats, a shy, slow, gentle fellow. Then there are honest Liljeberg the boat-builder, the wise Madame Nygard, the malicious storekeeper, a little horde of village children, and Katri's dog. Nameless, silent and yellow-eyed, the dog is yellow-eyed Katri's creature. And she flatters herself on her own wolfish superiority to other people: "My dog and I despise them. We're hidden in our own secret life, concealed in our innermost wildness."

No one in the village seems to be married, and the relationship that will form between the two solitary women, Katri and Anna, is not sexual, though it is intensely passionate, fiercely unstable, destructive and transformative. Anna, far wealthier than Katri, keeps her parents' house piously unchanged, and illustrates little books for which the publisher provides the words. Her paintings are marvellously truthful depictions of the forest floor, patterns of leaf, twig, moss, lichen . . . to which she adds the cute bunnies of the publisher's texts. She spends much time answering letters from her child readers, and none in looking after her business interests. She sleeps, sleeps all winter until spring comes and she can see the living ground and paint it.

Wolfish young Katri, determined to provide security for her brother, and also the fishing boat that is his one heart's desire, fakes a robbery of Anna's house in order to make her afraid to live alone, and pushes her way into Anna's service and confidence. Before long she appears to be in full control and has thrown out all the old furniture and the comfortable lies that let Anna sleep. But Anna, awake now, is not the bunny-rabbit she seemed, any more than Katri is truly the wolf. The unfolding of their story through vivid contrast and interplay of truthfulness and deceit, purity and complexity, ice and thaw, winter and spring, makes the most beautiful and satisfying novel I have read this year.

 This article was amended on Monday 14 December 2009. The original referred to Tove Jansson as Swedish. This has been corrected.

In 1962 Tove Jansson published a story for children called "The Spring Tune", featuring Snufkin, the peripatetic musician of the Moomin stories. "'It's the right evening for a tune,' Snufkin thought. 'A new tune, one part expectation, two parts sadness and, for the rest, just the great delight of walking alone and liking it.'" As he settles down to compose, he is disturbed by a small creature, a "creep", which rustles out of the undergrowth, declares its admiration for the famous Snufkin, asks him a lot of questions, and demands attention and comfort.

Meanwhile, the tune, which until then had been forming itself out of the noises of forest and brook and the slow revelations of the season, disappears. Snufkin has to wait for it to come back. Never underestimate Jansson, who never ever underestimates her reader. This story for eight-year-olds is a sharply pertinent discourse on the relationships between art, nature, fame and identity, a discussion of the place and role of the artist and of the mysterious sources of creativity.

It could be said that everything she wrote is, in one way or another, about the creative interactions between art and reality or art and nature.

Tove Jansson was born an artistic child of bohemian Finnish artists. Her mother, Signe Hammarsten, was one of Finland's best-known artists, designers and book illustrators; her father, Viktor Jansson, was a celebrated sculptor. Jansson herself became well known in her 30s for her Moomin tales and illustrations, which eventually made her world-famous. Because she was and is so recognised for her children's literature, her adult fiction, which she began writing in her early 50s (she died in 2001, aged 86) has tended to be overlooked, but in her last three decades the 11 books she wrote were all for adults. The UK republication of The Summer Book (1972) in 2003, followed by a selection of her short stories, The Winter Book, in 2006, and the first publication in English of her final novel, Fair Play, in 2007, has been revelatory for her English-speaking readership. That there can still be as-yet untranslated fiction by Jansson is simultaneously an aberration and a delight, like finding buried treasure, especially when the translator is as well suited to her resonant, minimal style as Thomas Teal (who was also the original English translator of The Summer Book in the 1970s). The True Deceiver is another fortunate first, and it is an unassuming, unexpected, powerful piece of work.

If the Moomins are Jansson's most celebrated legacy  a community of inventive, big-nosed, good-natured beings who survive, again and again, the storms and existentialism of a dark Scandinavian winter through simply being mild, kind, inclusive and philosophical  what will happen when a real community is put in its place? What will the outcome be when Jansson tackles, naturalistically, the life of a tiny hamlet in a dark, wintry landscape  and in a book so close to real local life that the original Swedish publication carried a disclaimer saying it was in no way based on any real place, nor its characters on anybody living?

A novel about truth, deception, self-deception and the honest uses of fiction, The True Deceiver is almost deadpan in its clarity and seeming simplicity, and is at heart one of her most mysterious and subtle works. First published in 1982, it was her third novel specifically for adults. Her biographer, Boel Westin, records that she had great difficulty with it. "Its unsparing view of life," Westin comments, "is, in fact, one of the characteristics of her adult books." Jansson herself commented on how "stubbornly, labororiously" she had worked on it. There's no doubting the oppressiveness of the conditions under which her characters have to live and work. "The winds had risen. It pressed snow against the windows with a powerful whispering that had followed the people of the village for a long, long time. Between squalls there was silence."

It begins with the disarming simplicity that characterises the whole novel. "It was an ordinary dark winter morning, and snow was still falling. No window in the village showed a light." It's a book about a dark place, where snow creates a kind of claustrophobia, where "paths filled with snow as quickly as they were shovelled out", and where "people woke up late because there was no longer any morning". By paragraph two the censoriousness of small community life has set in. "It's still snowing and there she goes again," the unnamed narrator comments about Katri. Katri and her brother, Mats, are clearly not liked in the village. He's too "simple" and her eyes are the wrong colour. Worse, they aren't properly "local".

The novel's voice is flat and exact, a kind of reportage, which shifts, seamlessly and suddenly, into Katri's own voice, making it unclear who the first narrator is and unsettling all notions of objectivity. By the end of the chapter, we know that this book, concerned with locality, money, winter, wildness, social unacceptability and power, will also be about whether there's such a thing as objectivity. Objectivity and truth are Katri's obsessions. Her refusal of social niceties, her honesty, her silences and her bluntness have made the villagers uncomfortable and deeply hostile towards her, but made her peculiarly trusted and given her a great deal of power in the community.

Is this also going to be a book about class and hierarchy? Within five short pages, Katri is standing looking at the local big house, which surreally resembles a giant rabbit's face, and is owned by an artist, Anna Aemelin, who lives there "all by herself, alone with her money". Her motives are clear: she and Mats are going to move into that house. The book begins on the projected standoff, dog versus rabbit, "the real story of Anna and Katri"  in other words, the standoff of "real" versus "story". At its heart is a battle that promises to be savage.

Katri wants an obliterating purity  "I wish the whole village could be covered and erased and finally be clean"  she is a personification of wintriness. Her opponent, Anna Aemelin, has no foothold on winter and is a being particularly associated with spring. "It was winter, and she never worked until the first bare earth began to show." Her art is dependent on the spring, and it almost feels, sometimes, as though the spring may be dependent upon her art. She's also a person practically disconnected from the village, an ageing child living in a veritable museum to her parents, and a famous artist, who draws forest floor pictures known the world over for their authenticity, then takes these "implacably naturalistic" pictures and adds lots of very unnaturalistic flowery rabbits, for which she is equally world-renowned, especially among small children. Who is the true deceiver here? And how does deception relate to truth? The novel, with its village full of mundane cheats and charlatans, is a philosophical confrontation between Katri's cynicism and Anna's aesthetic sensibility. Is there such a thing as kindness? Or is there only "the whole sloppy, disgusting machinery that people engage in with impunity all the time everywhere to help them get what they want, maybe an advantage or not even that, mostly just because it's the way it's done, being as agreeable as possible and getting off the hook"? What are flowery rabbits (or, it might be added, Moomins) actually for? Or is it Anna who's right, that the paying of attention to people's needs, though "a pretty rare thing," is a natural and uncynical part of being human? She knows what is expected of her, and she acts on it, just as she knows her own lie and finds it tiresome. But "Things are not always that simple." Katri, on the other hand, knows exactly what "simple" means. She has seen and destroyed the snow figures the village children have made in spiteful likeness of her and her "simple" brother.

Jansson's own texts are always honed to perfection, given a lightness that proves deceptive, an ease of surface which, like ice over a lake, allows you rare access to something a lot riskier and more profound. "Rarely do books give as clear an impression as yours that they have simply matured to the point of inevitability," Jansson's editor at Bonnier, her Swedish publisher, wrote to her when she was struggling with difficult work; in many ways, The True Deceiver is a book about artistic maturation as well as human coming of age.

Is this an autobiographical portrait? Jansson herself commented, at the time of The True Deceiver's first publication in Swedish: "Every serious book is a kind of self-portrait." This overexcited reviewers, who decided to see the book, a subtle and calibrated work, in the simplified and reductive terms which "autobiographical" almost always means. But the hopelessly innocent Anna Aemelin, totally at a loss with the commercial spinoffs from her flowery rabbits, is laughably far from the sharp-eyed Jansson, who could write so acridly and merrily (as she did in her short story "Messages") about the flurry of requests that came in from companies and individuals concerning her "product". Jansson knew the responsibility and surreality of her position, which could result in a request like the one from a company that wanted to use her tiny anarchist figure, little My, on "a discreet new mini sanitary towel" (she said a discreet no) directly alongside one from a reader asking for a drawing of Snufkin "that I can have tattooed on my arm as a symbol of freedom".

One of its most haunting moments is when Anna, looking through reams of her parents' old correspondence, trying to find a portrait of herself as a girl, discovers that she was hardly there. She realises that she became "a painter of the ground" only after both her parents were dead and buried in it. It is a deeply poetic work, and such images, like that of the dog that finally runs mad, or the pile of rubbish left on the surface of the frozen lake  all the piled-up ephemera of Anna Aemelin's life, which will sink when the spring comes and the ice melts  are pervasive. On the surface, this is very much a book about how to survive, as well as how to deal with what surfaces in lives, over time.

The True Deceiver is the opposite of charming  and deliberately so. But this novel's presentation of itself as a tough and unresolving work is a kind of deception in itself. "There are no real answers to what is right and what is wrong," Boel Westin concludes. That's one possible reading of the novel. But look at its deep understanding of human surreality and sadness, and at Jansson's vision of the epic qualities inherent in all small things. Though meticulous in its rejection of sentimentality, it demonstrates, alongside all the cruelty, a wealth of small, real acts of kindness. By the end, its two fixed protagonists, Anna and Katri  the two opposite poles of its "real story"  have learned to shift position. This change doesn't come without fracture  ice will break in the melt. All the same, at the end of this mysterious novel, both women have changed their old tunes for new. It is one of Jansson's most deceptively quiet, most astonishing compositions.