Check out some of these links to find a discussion to join:
Book Clubs Resource.com  a complete guide to book clubs and reading groups with an outstanding collection of links and information for readers, including information about saving with discount book clubs. Whether you're looking to start your own reading group, participate in one online, or just learn more about what a book club is, you'll make the best of your book club experience with Book-Clubs-Resource.com.
Book Spot  - book discussion links
Book Discussion Groups/Sites  - list of online groups created by San Antonio Public Library - dated but some still work.
See Genres of Interest  for additional specific groups to join.