Mr. Monster is a 2010 young adult thriller novel, the sequel to I Am Not a Serial Killer by author Dan Wells. It is the second book in the John Wayne Cleaver trilogy. The book focuses around the dual threats of the conflict between John and his darker side, called "Mr. Monster" (which was also one of the aliases employed by serial killer David Berkowitz, aka "the Son of Sam"), as well as the emergence of a second serial killer in Clayton County.