AllReaders.Com. You can specify specific types of plots, settings, or characters.
Bouricius, Ann. The Romance Readers' Advisory: The Librarian's Guide to Love in the Stacks. 2000. Another copy is also available electronically.
Herald, Diana Tixier. Genreflecting: A Guide to Reading Interests. 7th ed. 2013.
Ramsdell, Kristin. Romance Fiction: A Guide to the Genre. 2d ed. PN3448 L67 R37 2012.