An excellent post apocalyptic horror novel. Far ahead of its time. It prefigures not so much vampire literature but the future zombie literature. The ending of the book is what makes it rise so far above the rest of the pack and it is what was left out of the movie. It is that ending which is clever, unexpected and deep, that makes the novel so powerful.
These kinds of genre novels are not really properly defended as the important works of literature that they can sometimes be. A very good book and possibly one of the hundred best books of the 20th century.