One of the two great old school vampire take over novels. The other being Salem's Lot by Stephen King. Although they deal with the same concept they are very different.
Where King's masterpiece sets the action in a small town and makes the threat more confined and intimate (and scarier), McCammon's vampires attempt to take over Los Angeles with dreams of turning the whole world into the undead. I think the smaller version of the tale works better.
That being said, any time you get to read a book by McCammon you are guaranteed a great time. They Thirst is well written, the characters are interesting, the plot hums along at a nice pace that eats up the 600 plus pages before you even know it. The vampires are nasty, with a Hammer Horror Film/Universal Pictures quality about them that I found very satisfying, and there are some very chilling scenes----particularly one set in a vampire ridden tenement house. He even plays around with the standard vampire legend a bit which is interesting.
This is an early work by McCammon, to be contrasted with the more mature and masterful The Wolf's Hour. Definitely not to be missed as it is a great vampire tale, but I enjoyed it also because I could see the seeds of the greatness that came to bloom in his later works.