They Psychopath Next Door by CarsonFaircloth

DESCRIPTION

Cooper Daniels lives next door to a psychopath. He just can't prove it...yet.When the most popular girl in high school is murdered at a Halloween party, Cooper's suspicions become a reality. He knows exactly who did it. But how is he supposed to prove his charming neighbor is a killer?Meanwhile, Calla Parker is waking up with blood under her nails, and she has no idea why. Unable to remember the night before, Calla begins to suspect herself of murder. She's relieved to find out the police have zero evidence to go on. Yet she can't help but shake the feeling she's being watched - very closely. And she knows why.

Cooper Daniels knows her secret. 

*Please read the book's individual trigger warning if one is included.