Young Cam Jansen and the knock knock mystery Adler, David A. Popularity → Current 15 Peak 25 Niche 45 Eric's Grandma Minnie and Grandpa Max keep hearing someone knock at their door, night and day, but find no one there, so Cam tries to solve the mystery using her amazing memory. Penguin Young Readers, an imprint of Penguin Group (USA) LLC 2015 31 pages