If you are really trying to keep this to one category/genre per book, I think Catlady's suggestion is the best. You can find lists of genres (
1,
2,
3) and cull them down to what works for you.
Personally, I do something more like jhowell and assign several categories/genres to each book. I used to try to use just one genre but there are way too many books that have elements from multiple genres.