Genres are very difficult to list. Most books don't fit cleanly into one single genre. Many authors also write in various different genres, so it can make things a bit confusing when you see an author like Stephen King with books in over 10 different genres.
We do plan on using genres in regards to books and authors at some point. It will be a big part of our recommendations engine. We just haven't decided the best way to handle it yet. So genres are coming for sure - we just want to make sure we handle the implementation of it perfectly.