Swallows and Amazons is the first in a series of books by Arthur Ransome, written in 1930, about, basically, children sailing and having adventures.
The Swallows are the four older children of the Walker family (based on friends of Ransome's, the Altounyans) John, Susan, Titty and Roger. They sail a clinker-built dinghy The Swallow in the Lake District in northern England.
Here, they act out fantasies of the high seas, with John as Captain, Susan as Mate, Titty as Able Seaman and Roger as Boy. They sail off to a desert island, where they find hostile natives, the evil Captain Flint (and his lost treasure), and pirates, in The Amazon.
The pirates are Captain Nancy and Mate Peggy Blackett, their ship, another dinghy.
The two crews struggle for supremacy, finally forming an alliance after Titty outwits the Amazons, and they surrender. Then, they join forces to take part in a combined offensive on Captain Flint in his ship in Houseboat Bay.
Of course it's all imagination. The children are just camping on Wildcat Island for a week (back in the days where the world was less threatening, or less paranoid, and a twelve year old was considered adequate supervision). Captain Flint is the Walker's Uncle Jim, trying to write a book on his houseboat, the character based on Ransome himself, and his treasure is a typewriter and manuscript mistakenly stolen by inept thieves. The natives are simply the other adults on the island --and the children buy supplies from them at need. But that doesn't matter -- it is real to the children, and therefore, real to the reader. Every child plays these games, and every adult has been a child who does.
It's a wonderful read, both for children and adults -- each of the characters is an individual, and believable -- Ransome's children are not the adult idea of what kids should be, like Blyton's, they are real people, something very hard to achieve from the distance of maturity. The sailing sections are knowledgable, the action is involving, exciting, and fun. Highly recommended, and a good book for reading aloud.