The Vampyre by John Polidori, I was pleasantly surprised by Carmilla. This is another piece of early vampire literature -- predating Dracula, which is often considered the father of the genre -- which I decided to read as part of my research for the vampire series I am currently writing. Unlike The Vampyre, however, it was told in a friendly, conversational narrative by the main character, Laura, who is being preyed upon (without her knowledge) by the vampire Carmilla.
Carmilla is unique in several ways. Not only does it develop the characters very well, especially in comparison to The Vampyre, it also portrays a romance between the vampire Carmilla and the narrator, Laura. For Victorian times especially, it's a little risque, with its suggestive descriptions of kissing and necking.
More importantly, though, you can see the birth of the genre, and the evolution of Dracula. Modern readers of vampire fiction will know what is going on long before the narrator does (though in those days, the reader probably would not have realized that Carmilla was a vampire). It's a short book, only about 100 pages according to my Nook, but it's well worth reading if you are interested in the roots of the vampire genre.