Small Group Vital Signs: Seven Indicators of Health That Make Groups Flourish by Michael Mack. This is a great book. It shows you how to take a group beyond fellowship and Bible study.
Holy Conversations: Talking about God in Everyday Life by Richard Peace. Years ago, Peace wrote a book entitled Small Group Evangelism. Here he has re-applied these principles in a simple 12-week format that can be used in groups.
Evangelism Outside the Box by Rick Richardson. Great book that reframes evangelism around relationships.
Missional Small Groups: Becoming a Community that Makes a Difference in the World by M. Scott Boren. For almost 20 years now I've been asking questions about how small groups can live in a way that naturally and organically changes the world. This is what I discovered.
101 Ways to Help People in Need by Steve and Janie Sjogren. This is a compilation of ideas with practical tips and guidelines for serving people "with no strings attached."
For pastors and small group champions who want to develop a system of groups that make a difference in their communities:
Launching Missional Communties by Mike Breen. This book speaks about a very specific form or model which they call a "missional community." This may or may not be the strategy that you adopt. Either way, it lays out some incredible principles.
MissioRelate: Becoming a Church of Missional Small Groups by M. Scott Boren. In this book, I deal with over-arching principles that will help you develop a system of groups that make a difference. The goal is to develop one that fits your specific tradition and local situation. In other words, these principles apply across many different specific strategies.