Okay, this is a first for me: a book that the author wants people to forget. Robert McCammon now forbids the reprinting of his first four novels, of which They Thirst is the fourth. He says the books don't represent his best work, that he was just a kid when he wrote them, and that he was "learning to write in public." He's certainly entitled to his opinion.
My opinion is somewhat different. I like it a lot. Vampires are everywhere now. You can't go into a bookstore without bumping into a vampire book every time you turn around. But back in 1981 when They Thirst was published, it was hard to find a good vampire book. It was hard to find any vampire books at all, for that matter.
Joe Lansdale recommended McCammon's books to me, and if I recall correctly, he likes this one quite a bit. It has everything: a serial killer known as "The Roach"; a 700-year-old vampire who's the King of the Undead and who's decided to go to war with humanity, starting with the conquest of Los Angeles; a Hell's Angel who's going to command the vampire army; a teenager who loves horror movies; and so on.
There are no nice vampires here. They're the spawn of Satan (did I mention that he's in the book, too?), and they're pure evil. The book, however, is pure fun. McCammon doesn't want it reprinted, as I mentioned, and I suppose he'd prefer that you not read it. So I won't even mention how cheap copies used copies are on the 'net.