Swallowdale by Arthur Ransome

A summer of adventure with the Swallows and Amazons. The Walker and Blackett children survive a shipwreck, discover a secret valley and cave, and go on a thrilling mountain hike.

On summer holiday, the Swallows (John, Susan, Titty and Roger Walker) and the Amazons (Nancy and Peggy Blackett) meet up on Wild Cat Island. Unfortunately, though, the Amazons have a problem: their Great Aunt Maria has come to visit and she demands that the Amazon pirates act like “young ladies.” Things get worse when the Swallows discover a very high hill that just begs to be climbed...