Paradise Valley by C.J. Box

Cassie Dewell, Chief Investigator for the Bakken County Sheriff's Department in North Dakota, is extremely close to capturing the serial killer nicknamed the Lizard King. However, he gets away and Cassie ends up losing her job. When Cassie finds out that one of her son's friends, Kyle Westergaard, has gone missing near the time that the Lizard King was in the area, she becomes concerned for Kyle's safety. Cassie agrees to go look for Kyle as a favor to Kyle's grandmother, Lottie. Does this mean that Cassie will finally be able catch the Lizard King once and for all? Paradise Valley is the fourth book in the Cassie Dewell series and shows Cassie's transition from being a member of the police force to private investigator.