Violets are Blue by James Patterson (Alex Cross #7)

Detective Alex Cross is drawn into a disturbing investigation after two people are brutally murdered in San Francisco in circumstances that appear to have a bizarre connection to vampire rituals. As Cross follows the trail across several American cities, he becomes involved with a secretive world of vampire role-players and increasingly disturbing crimes, while simultaneously being pursued and threatened by his long-standing nemesis, the Mastermind. With his professional investigation becoming increasingly personal, Cross must balance the danger facing those closest to him with his determination to uncover the truth. The result is a fast-paced crime thriller combining murder, psychological suspense, police investigation and a sinister underworld of people obsessed with vampires.