Blood Rites is the sixth installment in the series, published in August 2004. It is available in paperback and in the Sci-Fi Book Club onmibus Wizard At Large with Dead Beat.
The following blurb is printed on the back cover:
For Harry Dresden, Chicago's only professional wizard, there have been 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 walking plant monster. Still, there's something more troubling than usual about his newest case. The film's producer believes he's the target of a sinister entropy curse — but it's the women around him who are dying, in increasing spectacular ways.
Harry's doubly frustrated because he got involved with this bizarre mystery only as a favor to Thomas, his flirtatious, self-absorbed vampire acquaintance of dubious integrity. Thomas has a personal stake in the case Harry can't quite figure out, until his investigation leads him staight to Thomas's oversexed vampire family. Harry's about to discover that Thomas's family tree has been hiding a shocking secret: a revelation that will change Harry's life forever.