Even though it has been out almost a year, “Groundswell: Winning in a World Transformed by Social Technologies” ($19.77 plus S&H, click to purchase on Amazon) by Forrester analyst Josh Bernoff (blog, Twitter handle) and former analyst Charlene Li* is still very interesting and relevant. I had picked it off the bookshelf after writing about the one year anniversary of the Analyst Twitter Directory and found myself nodding in agreement as I reread it.
* Charlene (blog, Twitter handle) left Forrester Research in 2008 and has since launched the Altimeter Group.
Highly recommended.
Building a Community 
Thanks for the plug! We spend an awful lot of time with corporate clients on these ideas.