Sort Of Books published their first Moomin story in 2001, the year of Tove Jansson's death. The Book about Moomin, Mymble and Little My was exquisitely produced, with a witty and lyrical translation and Jansson's own illustrations. Not long after, her first novel for adults, The Summer Book, introduced a new readership to her work with this elegant and darkly funny tale of the friendship between a bereaved girl and her grandmother over one long, light-filled Finnish summer. Other novels and stories followed, tantalisingly spaced, and now, as we near her centenary, they bring us her very first picture book, The Moomins and the Great Flood, published for the first time in the UK.

Jansson was born in Finland, to a Swedish-speaking family. Her father was a sculptor whose work still stands in Helsinki's public spaces, her mother an eminent graphic artist and illustrator  her resourceful nature an inspiration to Jansson in much of her work. Jansson herself trained as an artist, only turning to writing as an escape from the deadening horror of Finland's war with the Soviet Union. Painting then seemed pointless, and she felt the need to write the words: "Once upon a time". But there were no princes or princesses in this, her first story. Instead, she used as her main character a small, angry cartoon figure she'd invented to amuse her younger brother. She named him Moomintroll, and gave him an eccentric, restless father and a warm, resourceful mother with an ever-present handbag.

Reading this book in the light of the suffering of the Finnish people in 1939 as they were caught up in the turmoil of their Winter War casts a different glow over what is essentially a classic adventure story. There are displaced persons (Moominpapa), divided families (the Moomintrolls), marching groups of soulless strangers (the Hattifatteners), and rewards for kindness rendered (the Stork, whose glasses were returned to him by Moomintroll).

We get a bit of Moomin history, too. I for one had forgotten that Moomintrolls used to live behind the stoves in people's houses and were driven out by the advent of central heating. It is at this point that the story begins, as Moomintroll and his mother, desolate at the loss of Moominpapa, who has wandered off with the Hattifatteners, set out to find a safe, warm place to spend the winter. As in all Jansson's work, nature is central, and her descriptions of it, wry and surprising, are the threads that lead you on. "Here and there giant flowers grew, glowing with a peculiar light like flickering lamps, and further in among the shadows moved tiny dots of cold green."

As Moomin and his mother make their way through the dark forest, over bogs and towards the sea, they meet a small creature more frightened than themselves, a girl with blue hair who has been living in a tulip, a vindictive and violent ant-lion, and a friendly sea-troll who thinks he may be a relation. There are sea serpents, the wrong kind of sunshine, and the Hattifatteners, who are both deaf and dumb and  Moominmama proclaims, when they fail to understand how desperate she is to find her husband  stupid too.

But for all their faults they cannot be ignored, and soon our little tribe of characters climb into the Hattifatteners' boat and travel with them across a raging sea.

"The boat sailed on with taut sails at a furious speed. Sometimes they saw a mermaid dance by on the crest of the waves, sometimes they glimpsed a whole flock of little sea-trolls. The thunder rumbled louder and the lightning ran criss-cross over the sky." Jansson is always wonderful on storms. For three decades she lived for much of the year on an island the size of a rock, only retreating to Helsinki in her 70s when her boat was broken up by bad weather and she realised she was too old to manage without it.

The sepia paintings that illustrate this section are among my favourites, with boiling waves and great, sheer cliffs, while the fine black and white line drawings that show Moominmama holding her son's head as he is sick, or fishing into her handbag for dry socks, are both witty and touching. But the last painting in the book, a landscape full of sunshine with a small, friendly house that looks reassuringly like a stove, is perhaps the most beautiful.

These are Jansson's own drawings, printed in their original hues, with all their humour and mystery shining through. It took six years, until the end of the war, for The Moomins and the Great Flood to be published, but when it was, Jansson described it as her "very first happy ending". How happy she would have been to see this glorious edition, I can only imagine.

 Esther Freud's Lucky Break is published by Bloomsbury.