Swallows and Amazons
This is one of the finest books for children, both boys and girls. In 1928, four children, aged 7 to 12 (?), whose father is in the navy, vacation with their mother and baby sister on a lake in the north of England. They borrow a 13' sailboat (the Swallow) from the farmer with whom they are staying, and camp on an island in the lake. They meet two girls (the Amazons), about the same age, and have realistic adventures together. The first in a series of twelve books, maybe the best kids' books ever written.