In 1972, Tove Jansson, creator of the Moomin characters, wrote her classic of island life, The Summer Book. The Jansson family, who were Swedish-speaking Finns, spent their summers on a tiny island in the Gulf of Finland, and the book describes this beloved place. Later in life Tove would move even further out to sea, spending more than 30 summers with partner Tuulikki Pietilä in a cabin on a remote, treeless islet called Klovharun.

The Jansson family’s island could be circumnavigated in four-and-a-half minutes. Island life was tough: food had to be brought by boat from the mainland, storms could prove dangerous, and the only link to the outside world was a radio. Practicality and self-sufficiency were vital, as was a sense of community; entertainment had to be created for oneself. Yet for Tove, this potentially claustrophobic world was endlessly stimulating. Reading The Summer Book 15 years ago I was struck by the way the island felt like my childhood house and garden: a vast world, full of imaginative potential. Rereading it now, in lockdown, the book feels like a survival guide.

Part-novel, part memoir, The Summer Book is a collection of 22 vignettes about a six-year-old called Sophia spending summer with her father and sculptor grandmother on a Finnish island: “A sanctuary for someone with work to do, a wild garden for someone growing up, but otherwise just days on top of days, and passing time.” Sophia was based on Tove’s niece, and the father on her brother, Lars. Very little happens: Sophia and her grandmother swim, row to nearby islands, build a tiny model of Venice, and beachcomb; a cat is brought in then taken away; the weather changes; Sophia sleeps in a tent. Around and beneath these events run conversations between the old woman and her granddaughter about death, creativity, right and wrong; all the while, Sophia learns to amuse herself, tolerate others, and manage her fluctuating moods and feelings.

At one point, a schoolfriend’s rare visit causes unforeseen tensions: “They had never realised that their casual island household was in fact an indivisible unit,” Jansson writes. “An island can be dreadful for someone from outside. Everything is complete, and everyone has his obstinate, sure and self-sufficient place. Within their shores, everything functions according to rituals that are as hard as rock.” The same can be be said of families, and The Summer Book deftly captures that world-building – heightened in this case by enforced and prolonged intimacy.

But while it may be small, the world Sophia and her grandmother share is endlessly rich and detailed. A clump of rotting, wizened trees is known as “the magic forest”; there’s a miniature wildflower meadow, a marshy pool that briefly becomes Venice, a creek and a potato patch; indoors, the attic, stove and guest room are also parts of Sophia’s imaginative kingdom. Moss clothes the island’s rocks, and everyone knows not to walk on it; the same wildflowers come up in the same places each year, and must be looked out for. What makes the small island sufficient to their needs is the quality and depth of their attention to it: “In July the moss would adorn itself with a kind of long, light grass. Tiny clusters of flowers would open at exactly the same height above the ground and sway together in the wind, like inland meadows.’

What also swells this small world is the fact that Sophia and her grandmother play in it, together and apart. Play can make anywhere numinous and magical. “Grandmother sat in the magic forest and carved outlandish animals. She cut them from branches and driftwood… ‘What is it you’re doing?’ Sophia asked. ‘I’m playing,’ Grandmother said.”

The pair create winding paths, make up stories about birds and passing boats, set up mysterious altars and draw pictures; they quarrel, worry about each other and collaborate. We call this “child-led play” these days; really, it just means taking children seriously, as fellow humans.

The Summer Book’s limpid style belies a deep psychological subtlety. It’s about how people can live close together for months with tact and grace, and about how rich and rewarding even a small world can be.

• Melissa Harrison’s novel All Among the Barley (Bloomsbury, £8.99) was UK winner of the 2018 European Union Prize for Literature