Cover of my 1974 St. Martin's Press Hardcover, 202 pages (originally published in French in 1973 according to the copyright page).
Weird Fiction Review called this "the most understated vampire novel ever written," and I agree. I have a feeling I would have enjoyed this more had I known nothing about it beforehand (and if it had a more ambiguous cover), but then it probably never would have come across my radar. Still, it's a good mystery, and very well-written. Full review to come.