The Other Mrs. by Mary Kubica
Married couple Sadie and Will; their neighbor Morgan; other protagonists Camille and Mouse
Warning! This is a SPOILER POST. If you haven’t yet read The Other Mrs., I suggest you turn back. Proceed at your own risk. Full review coming soon!
Camille and Mouse are Sadie’s alternate personalities; she suffers from DID. She doesn’t know, but her husband had figured it out and was using Camille to kill people. The husband, Will, had killed his high school girlfriend for cheating on him and covered it up as a car accident. When Will and Sadie moved to Maine, they unknowingly moved down the street from the girlfriend’s little sister, Morgan. Morgan suspected Will had killed her sister. Will convinced Camille to kill Morgan. Once Sadie found out she had DID, she figured out what Will had done. Her niece recorded Will’s confession as he tried to kill Sadie.