When Cam and Eric visit his grandparents' house, they find a mystery waiting for them. A Knock, Knock Ghost has been keeping Grandpa and Grandma awake: they frequently hear knocking sounds, but when they rush to open the door, they find no one there. Using her photographic memory, Cam cracks the case. As a mystery writer, Adler plays fair by dropping clues into the simply written, conversational text, and Natti also sprinkles them throughout the colorful ink-and-wash illustrations. A concluding Memory Game page challenges readers to look carefully at a particular picture and then answer six questions about it. An appealing addition to the Young Cam Jansen series.

Cam and best friend Eric visit Eric's grandparents, but Grandma and Grandpa are too tired to do anything--someone (or something) keeps knocking on their door and waking them up. Cam and Eric solve the mystery through their powers of observation. Simple language and sentence structure make this book accessible for beginning readers and may help lead them into the chapter books about these kids.

K-Gr 2 When Cam and her friend Eric visit Eric's grandparents, they learn of a mystery that causes Grandpa Max and Grandma to nap all day. At night, they often hear what they think is a knock at the door, but no one is there when they answer. Cam uses her photographic memory to identify the source of the knocking. Apples fall from the trees around the house, and they hit the roof, making sounds like someone knocking. Cam and Eric not only solve the mystery but also resolve the problem by picking the rest of the apples. Aside from the applesauce, apple pies, and apple cookies made from the harvest, Cam and Eric get satisfaction from eliminating the "Knock, Knock Ghost." The stories in the this series promote logical thinking and helpfulness for young readers beginning to enjoy simple transitional chapter books. Lindsay Persohn, University of South Florida, Tampa