I just finished reading Tamar Weiberg's book The New Community Rules: Marketing on the Social Web.
It's a little over 300 pages, but reads easily and provides a great reference tool for navigating social media sites such as Facebook and Flickr, and understanding blogs and why YouTube is so popular.
If you are in ministry and are not using social media, or just getting your feet wet, this book will really flesh out the why's behind everything and how to leverage social media towards your organization's goals.
I most enjoyed chapter 2-Goal Setting in a Social Environment, and chapter 3-Networking and Implementing Strategy. I've noticed that many ministries use social media haphazardly and dive into various platforms and spend resources on many different tools, without a clear goal or purpose.
Simply getting a handle on this area alone would dramatically increase many ministries and organizations use of social media.
Hope you buy it!