Mary Pope Osborne is the author of over forty books for young people, including novels, picture books, biographies, mysteries, and retellings of fairy tales and myths. She was the president of the Author's Guild, the leading organization for professional writers in the US, for two years and has won many awards for her books. She lives in New York City with her husband, Will, and their Norfolk terrier, Bailey, who sleeps on top of Mary's desk every day while she writes.

After a break from their trips in the tree house, Jack and Annie are delighted to find that it's back! Morgan le Fay needs their help, but they'll have to become Master Librarians first, and that means solving four riddles. For the first one, Morgan sends them deep into the middle of the ocean. Jack and Annie ride in a mini-sub and meet jellyfish, sea horses and an octopus. But there are some scary sharks in the deep and the friendly dolphins have to come to the rescue. . .