Arthur Ransome 
Swallows and Amazons (Swallows and Amazons Series #1) 

The ultimate children's classic – long summer days filled with adventure. John, Susan, Titty and Roger sail their boat, Swallow, to a deserted island for a summer camping trip. Exploring and playing sailors is an adventure in itself but the island holds more excitement in store. Two fierce Amazon pirates, Nancy and Peggy, challenge them to war and a summer of battles and alliances ensues. 'My childhood simply would not have been the same without this book. It created a whole world to explore, one that lasted long in the imagination after the final page had been read' – Marcus Sedgwick