I ususally have a long list of books I'm planning on reading. At other times I am stumped. Finding a good fiction book can be particularly difficult. Book Forager can help. You chose from a range such as: happy/sad, sex/no sex, optimistic/bleak, etc. You can also choose by setting, plot (success against the odds, for example) or the character's race, age sexuality and gender. It actually make quite good suggestions. You can also trick it with combinations that don't make sense such as happy and bleak.
http://www.branching-out.net/forager/forager.htm