FFB, THOSE WHO HUNT THE NIGHT, Barbara Hambly

This is the only novel about vampires I ever read and it was awarded the LOCUS Award for Horror. You would expect I would try Anne Rice next but she was too gothic for my tastes.

 

From wikipedia.  

Somebody is killing the vampires of London. Against the wishes of his fellow undead, Simon Ysidro, oldest of the London vampires, seeks the assistance of Oxford professor James Asher, former spy for the British government. Ysidro gains Asher's cooperation by threatening the life of his beautiful young wife, Lydia.

Unbeknownst to Ysidro, Asher enlists the help of his wife, a physician with a keenly analytical mind. Asher prowls the streets and crypts of London with Ysidro, entering the dark underworld of the undead, as Lydia combs property records and medical journals for clues as to who might have the means and the motive to carry out the slaughter.

Asher's theory is that the killer must be a vampire himself, one able to remain awake and active in the daytime. Lydia develops a theory as to how such a vampire might come to be. Together Asher and Ysidro travel to Paris to seek out the mythical eldest of all vampires, Brother Anthony, who might be either the killer himself, or the key to the killer's undoing.

What they discover is a threat to the living as well as the undead. A scientist seeking to artificially endow British agents with the powers of vampires is behind the killings. The scientist's son has been transformed into a hybrid vampiric monster who can walk in the daylight, but craves the blood of other vampires to arrest the degeneration of his body. Asher and Ysidro, aided by Brother Anthony, destroy the hybrid vampire and end the threat. Ysidro leaves Asher and Lydia in peace and vanishes from their lives.