Natural Law — Joey W. Hill

Mackenzie "Mac" Nighthorse is a highly respected homicide detective and a career cop. There isn't a dangerous situation that can back him down. However, the personal becomes uncomfortably entangled in the professional when he goes undercover in a high class BDSM club as a male submissive to find a Dominatrix who is winning the trust and then murdering her chosen partners. When his path crosses Violet Siemanski's, he realizes he's found a Mistress willing to be his ally in finding the murderess. But Violet is also going to shatter Mac's shields and make him question who he is and what he wants, a challenge more disturbing than any case he's ever worked.