The two most unflinching self-portraitists of the 20th century were Remedios Varo and Tove Jansson.

Big noses are just part of the connection. Varo turned her own outsized proboscis into an instrument of magic, a badge of outsiderhood, a carnival mask of sinister portent. Tove Jansson, who drew and painted herself endlessly, never prettified herself and put much of her identity into the character of a small band of creatures recently described as "hippopotami with scrotal noses".

The Moomins are not so much fictional characters as inhabitants of a world parallel to ours. Their autobiographical import is clinched when we find Moominpappa leaning on a classical pillar in the exact pose of Rembrandt's Self Portrait With A Fur Hat, and then see that Jansson has also painted herself, also furred, in the same attitude.

Animal skin was one of the totem objects of her life. In her teens, she made herself an alarming pair of fur trousers; cat, squirrel, lynx and rabbit pelts play a role elsewhere. It is as if Jansson brimmed with so much feral energy that she was always on the brink of transformation.

Boel Westin has been through pretty much every scrap of paper pertaining to Jansson's long life and well-loved work, and is better placed than anyone previously to determine not just the origins of Moomintroll but how that moonlit world pertains to ours.

However benign the Moomins now seem, they came out of dark places. It is fascinating to find that the very first versions were not white but black. An early, undated watercolour shows a lighthouse seen from above, with a black Moomin rowing out into the darkness.

Its public beginnings were in the "snork", a tiny animal figure that became Jansson's signature when she contributed political cartoons, some breathtakingly brave, to the satirical magazine Garm. Finland and Sweden, her two countries of formation, had very different experiences in the Second World War, which began when she was 25. Finland suffered a "Continuation War" with the Soviet Union. Sweden remained uneasily neutral, but with leanings toward Germany. Satirising Hitler or Stalin was deeply risky.

Both her parents were artists and enjoyed a brief idyll as sculptors/illustrators in Paris. Tove wrote about her father, Viktor Jansson, (always "Faffan") in Sculptor's Daughter, but her mother Signe (affectionately known as "Ham") was just as much a model, not just as an illustrator but as a balancer of life and art. Tove's output is staggering: books, comic strips, poetry (albeit unpublished), paintings, illustrations, diaries.

In nature, she was an almost perfect blend of father and mother, male and female principles. She shared their appetite for storms and mad wheezes, but also recognised that the storms of human history exacted a price. The Moomin tales, with their fateful comet and images of apocalypse and privation, come directly out of wartime experience.

While in some sense engaged to the fierce, energetic Atos, she fell "violently" in love with Vivica Bandler, the first of her lesbian relationships that culminated in a lasting partnership with Tuulikki Pietilä (or "Tooti"). Tove portrayed the couple as Thungummy and Bob, whose happiness is only threatened by the Groke, a figure who represents all that is anti-life and killjoy.

She lived with a sense that many readers thought "Tove" was a man. Psychoanalysts have always made hay with Moomintroll - Pappa Moomin's tail, which is characteristically tucked into his pocket, and the scrotal/phallic nature of those extraordinary noses.

Underneath all this, though, is joy and genius in unusual combination. Jansson's work grew out of dark times, but transcended them utterly. Her vocation was "making life into pictures and words", always with the proviso, uttered by Snufkin, that "life isn't peaceful".

This is a model biography, racy, unprurient, insightful, delightfully illustrated, true to its subject but also true to its own cleverly modulated narrative. One puts it down at last and says "O, to be a newly-woken Moomintroll dancing in the glass-green waves while the sun is rising", which is a wish alike for childhood and for age.