I read this book in the early 1990s. It's been a while, so my details may be a bit fuzzy.
I recall that the setting was in the late 19th century and a human detective is approached by a vampire to investigate the deaths of vampires. It turns out that another vampire is killing his own kind.
That's about all I remember about the plot. I got it from the Science Fiction Book Club back in the day, and I think it was one that got shipped to me, because I didn't send the card back in time. It was entertaining though.