If you are looking for a book to shove in your pocket and read secretly when no one is looking, then get a copy of Tove Jansson's The Winter Book.

It has to be a secret, because this book is full of secrets; the wild intense secrets of childhood, the obsessions of old age that cannot be shared because they cannot be understood by others, the solitary happiness that depends on nothing more than a bare rock and an open sea.

There's a story here about a little girl who notices that an iceberg is following her. In the freezing Finnish night, when everyone else has gone to sleep, the iceberg comes near the shore. Facing the little girl is a grotto with an icicle grill, just the right size for leaping into and travelling away. But she loses her nerve, observing that 'if one doesn't dare to do something immediately, one never does it.' Instead of herself, it's her father's flashlight she throws into the grotto, where it lights up the ice, eerie and green. Then the little girl shouts at the iceberg to GO AWAY, and gives it a push, as hard as she can. Away it drifts, green and glowing, out from the islands of the archipelago and into the open sea. The little girl goes back to bed, imagining it, shining and beautiful - for as long as the batteries last - and berating herself for her cowardice.

Tove Jansson would be careful to tell us about the torch batteries. One of the pleasures of her imagination is its practical matter-of- factness, set against the most wonderful excesses. Anyone who has read the Moomin Books will recall that Moomin Mama's handbag is always stuffed full of exactly what is needed for an accident or an adventure. The Muskrat's philosophising is gently mocked in every Moomin book, because all he can do is lie in a hammock and think great thoughts. He needs to be fed and looked after, of course, and the Moomins do that, knowing that life works best when someone can saw the wood, light the stove, and cook vegetables, as well as write a book.

That's what Tove Jansson used to do through her long life, the summers spent on a remote island off Finland. She was born in 1914 and died in 2001, and over the years, her Moomin Book have been translated into thirty-five languages.

I recently received a letter from Elizabeth Portch, who first translated Finn Family Moomintroll into English - and who I must have read as a child. She sent me a book, and promised me another, when she 'kicks the bucket', which she assures me will not be long, as she is 87.

Portch's cheerful spirit and practical application seem to be part of the Moomin legacy. Jansson can break your heart, and as the Moomin books develop, she often does just that - but the pieces are mended again, and without sentimentality. In A Winter Book, there is a wonderful story of a child who finds a giant stone and decides to roll it home. That's what the story is - just that, and I won't spoil the end, but the catastrophe is a typical of Jansson - utter disaster, somehow made into a new secret, a new precious thing, and not really a loss at all - or like the sea, something is swept away, and something else, unexpected, is washed up on shore.

It is as curious to think of the Moomin books only as books for children, as it is to think of A Winter Book only as a book for adults.

The truth is that Jansson had a sense of the world that translates across age, and can no more be hemmed in than the sea that lapped round her beloved islands.

I am bored rigid with fiction that can't be bothered with language or art, and disguises its inadequacies as 'realism'. It is always a relief to come back to a place where the imagination is so strong that whatever it tells you becomes real.

It is fitting then, that Ali Smith has selected these short stories, and written the Introduction. Good writers are no more plentiful than they have ever been, but here are two of the best.

I don't see the point of life if there is nothing to read. The practical things we can do ourselves - we don't need fiction to do them for us - but the imaginative things - yes that's what I want on a cold winter's night.