"Desolate Angel" by Chaz McGee

This is the first book in the "Dead Detective Series". It's about a detective who gets shot during a drug bust, but his spirit still lingers. He realizes that he was sloppy in his investigations, and he wrongfully charged a girl's boyfriend for her murder. Killings continue, and he tries to help the current detective know who the real killer is. It was suspenseful and interesting the entire way through!