Very few people know that Joseph Sheridan Le Fanu was one of the first writers in history to write a fictional story about vampires. Yes, he did that before Stoker shocked the world with his insanely famous Dracula. Carmilla is classified as a Gothic novella and was first released as a serial. The story is told by a young girl who was bitten by a female vampire, Carmilla.
The critics and the readers really loved it back when it was first published, and, to this very day, various directors present their own versions of this centuries-old tale. Obviously, the author didn't come up with a vampire story all by himself: he used some sources that were available in his time. But, he did put a lot of effort into making it an entertaining and exciting read, as opposed to most of the mystery fiction that was being released in the 19th century.
By the way, Carmilla, the main character of the tale, is actually also one of the very first lesbian vampires in English literature. Now, even though the writer doesn't say it directly, there's a lot of sexual tension between the vampire and the young woman that she bit. And, she only chose female victims, never attacking the men. She was gorgeous and had many abilities, including shape-shifting and walking through walls.
Plus, she liked to sleep in a coffin. It might sound like old news in the 21st century, but ~150 years ago Le Fanu's story raised a lot of eyebrows, to say the least. You could say that Carmilla is the godmother of all the contemporary sexy vampires, including the characters from The Twilight, The Vampire Diaries, True Blood and other mainstream hits. And that is why it's considered to be a classic!