Carmilla is a vampire novella by Joseph Sheridan Le Fanu, and perhaps one of the greatest in the genre, despite the overwhelming presence of Bram Stoker's novel, Dracula, which was strongly influenced by Carmilla. Though Carmilla was not the first vampire novel (the considerably older The Vampyre by John William Polidori probably holds that title), it was certainly an influential work.
Carmilla is everything that a vampire story should be: richly atmospheric, eerie, unsettling and deeply frightening to those of a nervous disposition. It was first published in a collection of short stories, In a Glass Darkly in 1872. Its setting is a parochial section of Styria province, Austria.
Carmilla, the title character, is the original prototype for a legion of female (and often lesbian) vampires. Though Le Fanu portrayes his vampire's sexuality with the circumspection that one would expect for his time, there can be little doubt that lesbian attraction is the main dynamic between Carmilla and the narrator of the story. Carmilla selected exclusively female victims, though only became emotionally involved with a few. Carmilla had nocturnal habits, but was not confined to the darkness. She had unearthly beauty and fantastic strength; she was able to change her form and to pass through solid walls. Her animal alter ego was a monstrous black cat, not a bat as in Dracula. She did, however, sleep in a coffin.
Carmilla has been the subject of a number of films. A film Et mourir de plaisir (Literally "And to die of pleasure...", but actually shown in England as "Blood and Roses") was made of Carmilla by French director Roger Vadim, with cinematography by Claude Renoir; it is perhaps one of the greatest of the vampire genre movies, and, predictably enough in a film by Vadim, thoroughly explores the lesbian implications behind Carmilla's selection of victims.
The British pulp horror movie house Hammer Films also had a go at Carmilla in Lust for a Vampire, its sequel Twins of Evil and The Vampire Lovers, Ingrid Pitt appearing in these as the anagramatically renamed Mircalla. In all there are about twelve instances of films based on the Carmilla story.
Though Carmilla was a seminal work for the genre of vampire fiction, there is also a modern tale that directly incorporates Le Fanu's character. Carmilla: The Return, written in 1999 by Kyle Marffin, begins in 19th-century Austria but follows Carmilla's life into 1990s Michigan.
(I have not read this book. It was not clear from the reviews I read whether Carmilla: The Return is a proper sequel of a re-telling of the original Carmilla.)