It is important to consider that the book was published in 1872 and was the first (?) vampire story. Being a short book, there isn't much development but the story held promise. The style is poetic, with long flowing sentences. The writer seems to like the word picturesque as it was used quite a few times throughout the book.
Reading about a female vampire preying on young girls was interesting. There were hints of more than friendship between the vampire and the narrator. Can we call it courtship or something more? Definitely intriguing, considering the time it was written.
The book isn't outright horror or doesn't have any gore. It is subtle, soft, and creeps around you with determination. (reminds of Carmilla's movements?)