Third in the Laws of the Blood series.
From the back of the book:
Selena Crawford is a Chicago homicide detective. She is also the unwilling companion of Istvan, chief of the Enforcers (an elite group of vampires who must serve and protect their secret community). By the letter of the law, a vampire can do whatever he wants with his companion--and most do. In Selena's case, however, Istvan wants as little to do with her as she does with him. And though they cannot deny the bloodbond that drives them together in passion, they do their best to avoid one another...
That is, until they both find themselves investigating the murder of a vampire. Now they must put their feelings aside to find the killer and maintain the secrecy of the vampire race. But when they find the killer and learn her motive, the threat turns out to be much more serious-and much closer to home.