Richard Matheson's I Am Legend is one of Stephen King's biggest inspirations as a horror novelist; that should say a lot just in itself. The book is a classic and today remains of the scariest vampire novels ever written. It's seen two film incarnations: The Last Man of Earth with Vincent Pryce and The Omega Man with Charlton Heston, with another coming out this year featuring Will Smith. If you want vampires, you go to Richard Matheson.
The story about Robert Neville, the last man alive in a world overrun by vampires, is a dreadful, heartfelt tale of survival. You can feel his sadness when he's all alone and his fear when he's not.
At under 200 pages, I think the book could have been longer, but you end up with a tremendous story that ends before it can play itself out.