Harvester of Bones. SPECTR Series 3, #4

Federal exorcist John Starkweather is consumed by the search for the truth about his past. A new lead takes him away from New Orleans, to confront the twisted man who experimented on him as a teen. Left behind yet again, Caleb and Gray throw themselves into the hunt for a murderous demon lurking deep in the swamp. But they can't shake their growing fear that the more John's memories return, the further he pulls away from them. As John digs deeper into the secrets surrounding the Center where he was held captive, the danger around him grows.
Because someone wants the truth buried-and they're willing to kill to keep it that way.