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One of Dean Koontz's Odd Thomas books? The first one was published in 2003.
I think it was part of a series, but I think I would remember if it was by Dean Koontz. I think that at one point in the book he got punched by the boyfriend of his ex-wife and at the end he was at a family cookout.
edit: I just found it. The book is called Perfect Circle by Sean Stewart
Edited by amazinglyenough
A book I read around the early 2000's. It takes place in the modern day and the main character can see and communicate with ghosts. He works as an Occult Detective and in the beginning of the book he is called to someone's house to check out a disturbance in their garage, and finds that it's the ghost of a teenage girl, and he tries to convince her to stop bothering the living inhabitants of the house. I don't really remember anything else about the book, though.
Edited by amazinglyenough