This novel predates the modern slasher film, so it's horror comes entirely from that which predates it, both the Victorian vampire novel and Edgar Allen Poe. Indeed, quite a few moments in the novel harken back to Poe.
The horror of this novel is multilayered and multi-fasceted, continually letting you grow used to the horror and the situation before you, only to turn subtly, exposing some new aspect that realienates you to the characters before you.
Forty years later, the writing in this novel still holds up, not yet fallen under its own cliches. Indeed, the cliches in this work still feel freshly buried, for this is the rotting soil from which the modern vampire cliche clawed itself.
Myself, I normally avoid horror novels. They just aren't my thing, yet I found Anne's writing solid, her characters well expressed and compelling, and the interrelationship of the characters continuously repellent. Indeed, at the character level is where the horror of this work best expresses itself. Told from a first person point of view by the vampire himself, his eternal anxiety provides the underlying angst of the work, but this is not the anxiety of a whiney-hiney, but the anxiety of self-loathing murder who has some sense of his own vileness.
If you're up for this sort of read, I greatly recommend this book. If you don't have the heart, then skip it.