When they follow Ms. Crowley to the back of the library where she disappears. Cleo and Evan discover a secret passageway to a hidden library full of books that have locks on them. One of them is open, and suddenly, they find themselves in the world of the book. There's a letter from their old librarian, who tells them to be very careful. If they complete the story with a happy ending, then they get to return to their world. If they fail, they get stuck in the pages of the book, just like Ms. Hilliard. Cleo and Evan find themselves in the 1800s, even wearing clothes from the period. They must assist failed detective, Artie and his bulldog Watson.
The mystery was well hidden. Eric Luper does a wonderful job at placing red herrings and clues for the reader. Cleo and Evan are very different from each other. Cleo is depicted as braver while Ms. Hilliard calls Evan "fragile." He's nervous about the adventure right from the beginning. But his smarts are a great asset to the team and he becomes braver as they go along.
As they look for the Musgrave Moonstone, they come across different characters and suspects. The banker, the young Musgrave daughter who is about to get married, the butler, the cook. It's like a game of Clue! This is the perfect for young readers who are fans of light-hearted mysteries. The Mysterious Moonstone is the first in a series that follows Cleo and Evan.