The The Discreet Eliminators series - The Pale House Devil

Ford and Neuland are paranormal mercenaries-one living, one undead; one of them kills the undead, the other kills the living. When a job goes bad in New York, they head west to wait for the heat to cool down. There, a young woman named Tilda Rosenbloom hires them on behalf of wealthy landowner, Shepherd Mansfield, to track and kill a demon haunting a mansion in remote northern California. As Ford and Neuland investigate the creature they uncover a legacy of blood, sacrifice and slavery in the house. Forced to confront a powerful creature unlike anything they've faced before, they come to learn that the most frightening monster might not be the one they're hunting...