This site has been around for a while, so you may have heard of it already. ReadingGroupGuides is helpful, because not only does it list discussion questions for thousands of popular book club titles, but it also has a list of what is the most popular, and how to get a book club going.
In addition, it also offers some opportunity for interaction, with polling questions, and becoming a formally registered book club. Authors, librarians and booksellers are interviewed as well, and there are giveaways and opportunities to win books.
So, the next time a patron asks you what their club should read, you can wow them by giving them a title, and a list of discussion questions to go with it!