AllReaders.Com. You can specify certain types of plots, settings, or characters.
Buker, Derek M. The Science Fiction and Fantasy Readers' Advisory: The Librarian's Guide to Cyborgs, Aliens, and Sorcerers. 2002. Also available in paper, Z688 S32 B85 2002.
Burgess, Michael, and Lisa R. Bartle. Reference Guide to Science Fiction, Fantasy and Horror. 2d ed. Reference PN3433.5 B87 2002.
Herald, Diana Trixier. Fluent in Fantasy: A Guide to Reading Interests. Another copy: IMC Reference 016.8-883 H531fl.
Herald, Diana Tixier. Genreflecting: A Guide to Popular Reading Interests. 2013. PS374 P63 R67 2013.
Herald, Diana Tixier, and Bonnie Kunzel. Strictly Science Fiction: A Guide to Reading Interests. Z688 S32 H47 2002.