Doyle, Michael, and David Straus. How to Make Meetings Work (Berkeley Publishing, 1993). This book gives practical guidance on how to structure and guide meetings.
Kaner, Sam. Facilitator's Guide to Participatory Decision-Making (New Society Publishers, 1996). This book outlines useful ways to think about decision making in groups. The "gradients of agreement" discussion is particularly helpful for guiding groups to recognize a spectrum of choices.