For people considering starting a small group ministry or thinking about re-engineering it, I would recommend the following books as good places to start. I actually pull these books out about once a year to refresh myself:
Building a Church of Small Groups byRuss Robinson and Bill Donahue (Willow Creek Model)
Dog Training, Fly Fishing, and Sharing Christ in the 21st Century by Ted Haggard (Free Market Model)
Creating Community by Andy Stanley (Northpoint's Closed Group Model)
Making Cell Groups Work by Scott Boren (Traditional Cell Model- this is more of a how-to navigational tool than a quick read)
From 12 to 3 by Joel Comiskey (the Infamous Principle of 12 Model in a more palatable form)
The Connecting Church by Randy Frazee (the Neighborhood Group Model)
Cell Church Solutions by Joel Comiskey is a pretty good overview of several different models.
NCC is actually a hybrid of all of these. It's actually kinda hard to describe our model because we are still in the process of discovering it. If I had to pick one, I would say we are primarily Free Market.
Other books I review periodically to check on the health of our groups include:
Seven Deadly Sins of Small Group Ministry by Russ Robinson and Bill Donahue
Walking the Small Group Tightrope by Russ Robinson and Bill Donahue