Magic Tree House 4: Pirates' Treasure! - Osborne, Mary Pope

Eight-year-old Jack and his little sister, Annie, are playing in the woods during their summer holiday, when they find a mysterious tree house full of books. But these are no ordinary books . . . And this is no ordinary tree house . . .Jack and Annie are in a treasure trove full of trouble! Stuck on a desert island, captured by the evil Captain Bones and his gang and a huge storm at sea . . . For a while, it looks like they might not get home safely after all. And they discover that their adventures are even more magical than they had first thought . . .