Blood Rites (The Dresden Files, #6) by Jim Butcher

Meet Harry Dresden, Chicago's first (and only) Wizard P.I. Turns out the 'everyday' world is full of strange and magical things - and most of them don't play well with humans. That's where Harry comes in. Harry's had worse assignments than going undercover on the set of an adult film. Dodging flaming monkey poo, for instance. Or going toe-to-leaf with a plant monster. Still, there's something troubling about this case. The Producer believes he's afflicted by an entropy curse - but it's the women around him who are dying. And Harry only got involved as a favour to Thomas, his flirtatious self-absorbed vampire acquaintance. Thomas has a personal stake in the case Harry can't work out, until his investigation leads him straight to Thomas's oversexed relatives. Harry's about to discover the skeleton in Thomas's family closet: a revelation that will change Harry's life for ever. Magic - it can get a guy killed.