The Summer Book by Tove Jansson

Absent of sentimentality, full of love and humor and wisdom, this is a tale about how much fun two people can have in the middle of nowhere, when they are practicing social isolation in earnest. It is pure loveliness.
The book distills the essence of the summer.   It tells the story of Sophia, a six-year-old girl awakening to existence, and Sophia’s grandmother, nearing the end of hers, as they spend the summer on a tiny unspoiled island in the Gulf of Finland. The grandmother is unsentimental and wise, if a little cranky; Sophia is impetuous and volatile, but she tends to her grandmother with care. Together they amble over coastline and forest in easy companionship, build boats from bark, create a miniature Venice, write a fanciful study of local bugs. They discuss things that matter to young and old alike: life, death, the nature of God and of love. 
Tove Jansson, the author, also wrote the Moomintroll comic strip and books .  She lived for much of her life on an island like the one described in the book, and the work can be enjoyed as her closely observed journal of the sounds, sights, and feel of a summer spent in close contact with the natural world.