Showing Kill You Twice by Chelsea Cain

Archie Sheridan should be recovering from his past run-ins with serial killer Gretchen Lowell, yet he's just as haunted as the day she let him go. But when a cyclist comes across a corpse in Mount Tabor Park on the eastern side of Portland, Archie suddenly has a new case to focus on.

Then comes a call from an unlikely source. After months of ignoring calls from a doctor at the mental hospital where Gretchen is supposed to be locked away forever, Archie hears that she might have inside knowledge about the new investigation. But is she bluffing just to get close to him, and can he risk losing his only lead?

One thing is for sure: Gretchen Lowell is back, and Archie must decide if catching a killer is worth facing his demons one more time.