Tove Jansson’s novel is bewitchingly simple: Sophia and her grandmother spend summers on a tiny island in the Gulf of Finland. the elements of their story are as simple as the beginning of the world: rock and rosebud, cesspool and juniper, the colors of water and sky. And as melancholy; there is loss here — Sophia’s mother, dead of illness, and the grandmother’s own waning health — though grief goes largely unspoken. In place of eulogy and complaint, Sophia and her grandmother practice the balm of attention. Their focus holds on the ephemera — cloud and down, root and weather — they attend to their changes, husband their possibilities. The vignettes that serve Jansson as chapters here are contiguous, not continuous, suffused with the serial disconnectedness of days of sun and rain. Migratory waterbirds come, and one dies of love; a storm rises in answer to a deliciously wicked prayer; crates of liquor bob in the ocean on midsummer’s eve. Jansson, who lived on an island like Sophia’s, became famous for her “Moomintroll” comic strip and books for children. Like the small forest creatures in those works, Sophia and her grandmother infuse their acts and days with a mythopoetic intensity. Disregarded and unnoticed by the wider world, they make stories to sustain themselves. And yet they learn that the worthwhile things are found alone and in quiet, as a cat pounces on a bright bird amid the flowers. Jansson’s small masterpiece is a summer book, to be savored with summer’s end.