Tove Jansson would have been 100 this week, an event that provoked a huge outpouring of affection. Much of this has to do, of course, with her timeless Moomin stories, but there’s another part of her life, an idea, that has proven to be just as enticing to readers: Tove’s island.

‘An astonishing number of people go about dreaming of an island’, she noted in her 1961 essay The Island. Half a century later, with the world more connected that ever, the idea of an island of one’s own is more compelling that ever: the isolation, escape and proximity to the elements speak to so many of us.

It’s something that Tove returns to again and again in her writing. In Moominpappa at Sea the restless patriarch uproots his family and leads them away from the valley to a remote inhospitable island. Little My describes it as being like a bit of fly dirt on the map. And in her autobiographical collection, The Sculptor’s Daughter Tove writes about her own childhood experiences, and her father who was most alive when dragging his family into the stormy seas around their island. But nowhere is the draw of island life better encapsulated than in the 1972 masterpiece, The Summer Book.

Here the father is a background figure – we are in the company of the women of the Jansson family: Sophia and her Grandmother, spending a summer together in glorious solitude on their small island house in the gulf of Finland. The pair are based on Tove’s own mother, Signe Hammersten (or Ham) whose death in 1971 inspired Tove to write the book, and her niece, the indomitable Sophia.

‘”I want to go swimming,” the child said. She waited for opposition, but none came. So she took off her clothes, slowly and nervously. She glanced at her grandmother – you can’t depend on people who just let things happen. She put her legs in the water.’

There aren’t enough books written about the relationship between child and grandparent. There’s an honesty that flows between these two generations at the extreme ends of life. But more often than not the relationship is used as a means of readying children for death – and no matter how beautifully executed, as in John Burningham’s Grandpa or Oliver Jeffers’ The Heart in a Bottle, the story ends up being overwhelmed by sadness.

Death is a constant in The Summer Book. Sophia’s mother has recently died, and so thoughts of mortality are at the forefront of her mind, as she quizzes Grandmother about when she is going to die. As the story progresses we see that the old lady is clearly near to the end – but instead of retreating it only makes the connection with Sophia stronger.

‘Wise as she was, she realised people can postpone their rebellious phases until they’re eighty five years old.’

There’s a glorious piece of bad behaviour instigated by Grandmother as she spies a new neighbour with a distastefully lavish new summer house. The pair take a boat over and ignoring the ‘keep out’ signs they land and break into the house – only to be caught out by the return of the neighbours. As the unlikely housebreakers take flight, comically, pathetically into the middle of the small island, the gap between child and grandparent vanishes altogether.

‘It was pure primitive flight, but she couldn’t think of anything better. Sophia leaped ahead, turned and came back, and ran around her in circles. The shame of being discovered on someone else’s island was enormous. They had stooped to the unforgivable.’

The Island is in many ways the reverse of Moominvalley, where the doors are always open to the constant flow of waifs and strays that Moominmamma welcomes with a piece of cake and a pot of coffee. On the island there’s a very different impulse, as Grandmother points out ‘We live here on this island, and people who come to bother us should stay away.’

The Summer Book was originally published without illustrations – Tove was keen to mark this out as an adult work, separate from her Moomin books. But several years later she relented and provided pictures for a new German edition, producing some of her finest – and final – black and white line drawings (available in this US edition).

The focus is mainly on the island itself, and when we do see Sophia and Grandmother it is only from behind.This conscious decision works perfectly in one of my all time favourite illustrations, showing the pair standing looking out at the sea. ‘Sophia followed along right behind her and saw how Grandmother carried the moon on her head, and the night became utterly serene.’

It might not be a children’s book as such, but the Summer Book is the most universal of all Tove’s works – appealing equally to young and old. For people like me, stuck somewhere in the middle, it succeeds in fulfilling a longing for that island of the imagination.