Karin Slaughter: Unseen: The Will Trent, Book 7 (The Will Trent Series, 7)

Special Agent Will Trent has something to hide. Something he doesn't want Dr Sara Linton - the woman he loves - to find out. He's gone undercover in Macon, Georgia and put his life at risk. And he knows Sara will never forgive him if she discovers the truth. But when a young Macon patrolman is shot and left for dead Sara is forced to confront the past and a woman she hoped never to see again. And without even knowing it, she becomes involved in the same case Will is working on. Soon both their lives are in danger.