The Mysterious Moonstone
Neither Cleo nor Evan like the new school librarian, Ms. Crowley, but they are very curious about where she disappears to at the back of the library, so one day they follow her--and find a secret door, a magical library full of locked books, and a letter from the previous librarian telling them that she is trapped somewhere between the covers of one of the enchanted books, and they must travel through the stories in order to save her.