Online communites exist in many forms. I'm particulalrly interested at the moment in the development of MOOCs - the cMOOC type in particualr - and building community is a vital part of the enteprise. In her book Amy Jo Kim demonstrates a great deal of wisdom and experience in her advice. From dealing with growth to creating presence and roles for users there is a lot of useful material here. While the recent book by Robert Kraut and company covers many of the same issues Kim's text is a better introduction for people new to community building on the web.