The Searcher by Tana French

Cal Hooper has retired from the Chicago Police Department and has moved to the remote town of Ardnakelty, on the west coast of Ireland. He spends his days mostly in solitude, rehabbing the home he has purchased and listening to music. Then he realizes that he is being watched, and soon uncovers the culprit--a teenager named Trey. Trey wants Cal to find out what happened to his older brother, Brendan, who disappeared six months ago. Cal doesn't want to get involved, but agrees when he realizes that Trey won't leave him alone until he agrees. Cal has to watch himself because everyone knows everyone else's business in this small town and, as a newcomer, he's not sure of whom he should be wary. On top of that, Cal is also concerned  that his investigation could bring trouble from Dublin. The Searcher is a leisurely-paced, finely-detailed story of a man who was looking for the quiet life in a small town, but found something else.