The Secret Zoo (Bk. 1)

Noah's sister Megan has disappeared, and he thinks that the Clarksville City Zoo has something to do with it. The animals are acting strangely, leaving cryptic messages for Noah and his friends Ella and Richie. Following the clues, they soon find themselves lost in a secret world more astonishing - and dangerous - than they ever imagined. Befriending polar bears, penguins, and rhinos on the way, Noah and his friends embark on a wild adventure to find Megan and bring her home.