While I agree that there is a certain amount of "if you build it, they will come" with regard to book writing, having a general idea of what category your book fits into will help others to find your book. It also helps when looking for an agent if that's what you're into. Not all agents want to represent the same types of stories.
That said, there is a lot of room out there for mixed genre. I've seen people asking where their book fits, because it has elements of several genres. For example, take Frank Hebert's "Dune". On the surface, you think SCIFI. But there's straight-up magic and a helluva lot of political intrigue. If I were Frank, querying that book today, I'd say it was a Sci-fantasy political thriller.
I wrote a Pride and Prejudice continuation that had vampires in it. Yeah, it fits into the Supernatural genre, and horror, too, but because it's based on a Jane Austen novel, there's romance there as well. There is NOTHING wrong with a book that straddles the line between genres. My book was Regency Supernatural Horror Romance. Also fits into derivative works and fan fiction.
So I would ask you, what are your elements? Is there horror? Magic? If you give us something to go on, I'm sure we can get you going.
