"The 9th Girl" is a mystery novel by Tami Hoag featuring detectives Elena Estes and Bobby Hallowell. The story revolves around the investigation of a series of murders linked to a group of young women, with the ninth victim being the key to unraveling the case. As the detectives delve deeper, they uncover dark secrets and connections among the victims. The narrative explores themes of friendship, betrayal, and the impact of past traumas, all while maintaining a suspenseful atmosphere that keeps readers engaged until the final reveal.