Sommarboken (The Summer Book) is one of Jansson’s earliest works for grownup readers, who were gradually to become her target readership. The diction of the book, its acuity of observation and its mundane sagacity, mark it out as belonging both to the Moomin universe and to that of adults. It is steeped in both, and it questions the difference between the two. In the latest English translation (2003) it is called a novel. This is not generically self-evident – the chapters are quite self-contained – but in accordance with the author’s original intention.

Tove Jansson was an avid traveller, who liked to write while abroad: she liked to focus her topics from a distance. Much of The Summer Book was written in New Orleans, in many ways an antipode of the skerry where it is set. It revolves around an elderly artist – the author’s mother, who had died in 1970 – and her six-year-old granddaughter, Sophia. They spend a summer together on a tiny island off Helsinki, in the Gulf of Finland. The island is a living presence in the book, on a par with the people on it. Summer passes exploring, talking about life, nature, everything but their feelings about Sophia's mother's death and their love for one another.

The Summer Book started out as a book which after Jansson’s highly successful career in draughtsmanship seemed almost ostentatiously non-illustrated.  Gradually Jansson provided it with drawings, and late yet authorized editions even exhibit photographs of the protagonists, including the island itself. There is a film version of it, and a CD read by the author (in Swedish; this particular chapter is lacking). The book itself is Jansson’s most widely read book for adult readers.