I love Reading Group Guides.com. They have over 3000 book discussion guides available, across all publishers.
Click on “Find a Guide” at the top, and you can choose by category, with plenty of fiction and non-fiction categories to choose from. You can also search by title or author. The best part of all? The “What to do When There’s no Guide Available” section, which offers generic questions for just about any type of book, from generic fiction and nonfiction to Biography, Current Events, and Politics. Every book discussion leader and member needs to bookmark this site!