Great Discussion Resources
Shared Inquiry Handbook
This is a free handbook published by the Great Books Foundation. It's a PDF that can be downloaded or printed out. This will help you understand the concepts and intentions behind a good discussion and how the right questions help a group get there.
Great Books Foundation
This is the parent organization's website.
Great Books Anthologies
The Great Books Foundation publishes anthologies for groups that provide highly discussable material, often with helpful questions included.