Skin trade - Laurell K. Hamilton

Once you tell someone certain things, like, say, you got mailed a human head in a box, they tend to think you're crazy.

Anita Blake's reputation has taken some hits. Not on the work front, where she has the highest kill count of all the legal vampire executioners in the country, but on the personal front. No one seems to trust a woman who sleeps with the monsters. Still, when a vampire serial killer sends her a head from Las Vegas, Anita has to warn Sin City's local authorities what they're dealing with. Only it's worse than she thought. Several officers and one executioner have been slain - paranormal style...

Anita heads to Las Vegas, where she's joined by three other federal marshals, including the ruthless Edward hiding behind his mild-mannered persona. It's a good thing Edward always has her back, because, when she gets close to the bodies, Anita senses "tiger" too strongly to ignore it. The were-tigers are very powerful in Las Vegas, which means the odds of her rubbing someone important the wrong way just got a lot higher...