Club Blood — Lauralynn Elliott

After Detective Kerry Harrison's partner dies, she is paired with Ethan James who is both good-looking and infuriating. They are assigned to a murder case in which Kerry and Ethan disagree about the guilt of the suspect. Further research into the case causes them to find that there have been similar murders in other places and that the suspects are all members of a secret club. Kerry finds out that Ethan not only knows most of the suspects, but also has knowledge of the club. Their partnership and friendship is tested when Kerry learns the true nature of the club... and Ethan's involvement.

Excerpt:

From her position, she could see some of the couples from a pretty clear angle and suddenly, she saw the man closest to her pull back a little from his partner's neck and there was a gleam of white sharp teeth reflected in the dim light. She drew a sharp breath as she saw, almost in slow motion, the man's teeth get closer to the woman's neck, and at that moment he sunk those teeth into her neck. She looked around at all the other couples and she saw teeth embedded in necks, shoulders, arms, and wrists. All around her there were moans and sighs of pleasure. It was like being in a nightmare orgy and she was shaking all over with terror at what she was witnessing. This couldn't be happening! It wasn't real. Maybe there had been drugs in the punch and she was hallucinating. This just couldn't be true.
What had Ethan brought her into?