City of Ghosts - Death Hunter Series, #1

A ghost collector grows deadlier than the spirits he hunts. Shane Ryan knows all about pain and suffering. A retired Marine gunnery sergeant, Shane has seen the worst humanity has to offer. He survived his ordeal, but his soul has been tainted by the darkness, leaving him with a permanent connection to the world of the supernatural. Shane sees the spirits of the dead, he hears their whispered cries of pain.
And it's a gift he plans to put to good use, when he travels to Detroit to investigate the death of one woman who saw through all the darkness in his heart. A woman who became his lover, many years ago. Shane is certain she was working a case, tracking down a deadly collector of the paranormal. And he's determined to use his abilities to force the spirits haunting the urban sprawl to reveal her killer.
But when he clashes with retired police detective Enoch Liddell, Shane realizes he's not the only one hunting ghosts. The two men soon find themselves locked in a supernatural game of cat and mouse. But it will take more than guts and guns to defeat this opponent, and his sinister ally.