"Flip this book to unlock another adventure!" -- Cover.

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.

The Spy's Secret: In their second trip through the enchanted library, the new key lands Cleo and Evan in the middle of a spy story, where their impossible mission is to stop a traitor who calls himself Viper--and they find themselves working with the replacement librarian, Ms. Crowley, whose exact role in their adventures is becoming increasingly mysterious.