Swallowdale (Swallows and Amazons #2)

The four children return to the lake for another summer camping on their island with their old allies, Nancy and Peggy, otherwise known as the Amazon pirates. But immediately disaster strikes when the Swallows find themselves marooned ashore by the shipwreck of their boat. But if they can't have the island, there's always Swallowdale, the secret valley, hidden from the world and containing an extra secret concealed within it.