This is a rollicking and vivid new translation of a lavish, beautiful picture book by Tove Jansson. Depending on how you feel about Jansson, that sentence will read either like a straightforward item of publishing news, or like the announcement of the discovery of a previously unknown Shakespeare sonnet. Or Beatles album. Or species of dolphin. Among the ridiculously excited might be Philip Pullman, Ali Smith, Esther Freud, a Japanese man who had Snufkin tattooed on his arm "as a symbol of freedom" and half a million Scandinavian girls who were christened "My" after another of Jansson's characters.

Jansson has a hold on her fans that is more than just nostalgia. Nowadays publishers repackage children's books for the adult market. Jansson did exactly the opposite. I remember being drawn to them by their charming, magical woodcuts and the cheery character names (Snufkin, Fillyjonk, Hemulen), then sitting down to read them in the library corner and finding myself plunged into a disturbing world where the physical unpredictability of weather and volcanoes was mirrored by the emotional fragility of the protagonists. It was as though Kierkegaard had come round on a play date. "You see how this Hemulen likes to collect and catalogue flowers? Well that's because he is trying to numb an underlying sense of existential panic. How funny is that!?" "Errmmm . . ." "And now look, Moominpapa is taking his family off on a sailing adventure. By putting his loved ones in danger he hopes to reawaken their sense of dependency. They are all hostages to his own emotional redundancy. Ho ho ho." In Moominland Midwinter, Moomintroll wakes up while the rest of his family are hibernating. His own dear, familiar home has been invaded by remote, uncommunicative winter creatures and even the Sun has disappeared. He's a stranger  even a ghost  in his own world. I don't think I've ever read a more compelling account of alienation. It's not surprising that the first Moomin drawing was meant as a caricature of Immanuel Kant. Nowadays I occasionally feel self-conscious reading children's books on the train. As a child reading Jansson it was just the reverse. I could sit there looking as though I was quietly reading a children's book while in fact staring into the abyss.

Jonathan Franzen famously said that the purest autobiography needs the purest invention. No one ever wrote a more autobiographical story than Kafka's Metamorphosis. Certain children's fantasies have extraordinary power because they are in effect metamorphosed autobiographies. Tom's Midnight Garden, for instance, is Philippa Pearce's attempt to retrieve her lost childhood home. The Moomin family is a version of Jansson's own bohemian, artistic upbringing. The Moominmama is equal to anything. She will greet the rummest, most unsettling stranger by laying an extra place at the table. Whatever disasters befall her family she has just what they need in her handbag.

The world they live in is dangerous and possibly overwhelming but they will survive in it by always making sure there is enough firewood and that the roof isn't leaking, and by being polite to strangers no matter how menacing they seem. Individual family members go off on their own adventures, have their dangerous secrets and bizarre opinions, but they always eat together and they share household tasks. She makes the family into an image of tolerance  a place where you are always welcome and never made to account for yourself. This is not only very beguiling, it's also very radical. In fiction  ever since fairy stories  the family home is the place you have to leave if you want to seek your fortune. In Jansson's world, Snufkin the wanderer  that image of freedom  comes home every summer. He never really says where he's been but he always comes back, and it's not really summer until he does come back. They are unbearably sad when he leaves in the autumn but they know he has to go and they trust that he'll return. I didn't know when I first felt the force of Snufkin's charisma that he was a kind of self-portrait by a lesbian artist who spent most of the year alone with her partner on a remote island.

Now I find it both poignant and uplifting that Jansson could describe so precisely and positively the relationship between a family and one of its members who chooses to live such a different life  how this difference somehow enriches the others, how they yearn to go off with her but know they can't, how they long for her return but need her to keep adventuring. In Moominvalley November a group of people like Snufkin come to the Moominhouse only to find that the family is not there. Jansson wrote this just after her mother died and its sense of absence is almost unbearable. Which makes this book  written five years later  all the more triumphant and joyous.

The heroine, Susanna, finds a pair of magic spectacles which transform the meadow in which she's reading into a series of terrible, beautiful landscapes  a jungle, a volcano, a dried-up ocean. Trying to make her way home, she's rescued by a balloonist and taken back to the safety of the Moominvalley. The pictures are breathtakingly beautiful  splashy, vibrant watercolours, very different from the poised, witty woodcuts of the novels. There are pink skies and upside-down birds and finally an overwhelmingly warm and welcoming picture of the Moominvalley seen from the air. Susanna looks very like Jansson's beloved niece Sophie and the balloonist who brings her out of danger back to the familiar rapture of the Moominvalley is TooTicky, a character based on Jansson's lover Tuuliki.

One of the many debts we owe to our favourite children's authors is the way that they alerted us  at an impressionable age  to various small pleasures. To this day I can give myself a sense of freedom and carelessness by setting out on a walk with a couple of hard-boiled eggs and some buttered bread  thank you, Milly Molly Mandy. William Brown pointed out the peculiarly sensual pleasure of beating your opponents into submission through extended bouts of spurious moral outrage. Jansson valorised coffee and pancakes and reticence and the mystery of others. But more to the point she showed me how it might be just those small pleasures that keep us together when we start to grow apart; that when words fail you can still express love by making sure the roof isn't leaking, the woodshed is full and the meal ready. Competence, manners and hospitality will keep us connected when nothing else will. We can tolerate any amount of difference, as long as we all agree to share the raspberryade.

Frank Cottrell Boyce's Cosmic is published by Macmillan.