The one thing you can always count on is romance being in the lead, and mystery/thriller coming in second. There's always some bleed over, some novels you can move from one area to another, but the rough numbers, strictly by genre, are these. Romance came in at 39.3% of the novel market in 2004. Also in 2004, mystery/trhiller came in at 29.6. SF/Fantasy/horror made up 6.4% in 2004. Mainstream came in at 12.9%, and various adventure fiction came in at 11.8%.
I've never seen a breakdown of how small SF/Fantasy/horror would be if you took out Star Wars, Star Trek, movie and TV tie in novels.
You can break things down another way and show that women's fiction makes up just a bit above 50% of the overall market, and you can separate some genres into other genres, but this is the rough genre picture. Or was in 2004.