I had the same thing happen and I was confused at first like you on what was going on. Realized that the two genre's, horror, poetry, were from the SN expansion pack that was to be released. I now have that pack and am able to write the books.
On writing best sellers, what traits your sim has can be a factor. I have a sim that I am using for an experiment, she has perfectionist, artistic, bookworm, c. whiz and ambitious. I am having her write one book of each genre, and then am focusing on the one I want her to specailze in, Drama. She has written 10 books so far; five best sellers, two hits and the rest are normal. Best sellers have been in horror, fiction, drama, romance and humor. Science Fiction and NonFiction have been hits, rest normal.
Don't know if that helps any, but thought I would share it just in case it does.