Ebook: Community Building on the Web : Secret Strategies for Successful Online Communities

I borrowed this from my local library and found it so helpful that I hunted down a used copy. It's not a technical reference, but it provides a great overview of the elements of online community, with authentic examples from existing communities (some of which, sadly, are now gone, but this was written before the bubble burst). Highly recommended if you are responsible for developing and supporting an online community, or if you are just interested in how thinking in online communities has been developing in the early days.