This book covers four main areas in regards to online communities: identity, social order and control, community structure and dynamics, and collective action.
Like many other texts on community, this book tends to focus on older technologies, i.e. Usenet, and MUDs/MOOs. That said, it contains a lot of good analysis done in these areas, and can provide good background for writing about online community. Note that the articles tend to be from the perspective of sociology. The strongest articles, in my opinion, were chapter 2, "Identity and deception in the virtual community," chapter 7, "Virtual communities as communities: Net surfers don't ride alone," and chapter 10, "The promise and peril of social action in cyberspace."
If you are interested in building a community or just in the ideas of online communities, this is probably not the best book for you -- it's pretty academic. Check out Jenny Preece's _Online Communities: Designing Usability, Supporting Sociability_ as an alternative.