Billie and the Secret Mystery Club are investigating their most serious mystery yet. Someone has taken something from the principal's office at school! Who would have done such a terrible thing? And when one of Billie's friends becomes a suspect, will Billie even know who to trust anymore? A Billie B. Mystery series: A slightly older Billie, her best friend, Jack, and other familiar friends solve mysteries at school and in the neighborhood, using their smarts and common sense to get to the bottom of things. Paperback book. 96 Pages. Ages 6 to 9 years.