detective fiction

Hi! New to this group. I am trying to find the name of a police fiction book. I bought it in the supermarket checkout about a year ago and loaned it to someone who claims they gave it back. It is part of a series. Here's what I remember: Police detectives searching for a killer. Possibly seen by a lonely teen who shares the main detective's taste in music and the detective gives him advice on learning to talk to people. The killer ends up being a police lieutenant who had his childhood abuse record sealed, which the detective receives and thus finds him. Name of book and or name of author would be appreciated.