Designing Social Interfaces by Erin Malone & Christian Crumlish is finally hitting the store shelves. It should be available on Amazon for direct ordering in the next day or so (still on pre-order when I checked).
I had the privilege of reviewing early versions of the book and wholeheartedly recommend the book.
Erin & Christian do a thorough job of cataloging a large number of design patterns covering all aspects of designing interfaces with a social dimension. It works on two levels. First it is an excellent reference book. With dozens and dozens of patterns it's easy to pick up the book and read any pattern in isolation of the others. But it also works when you come from a specific problem. Wonder about building community participation? Or the merits of tagging? Or just need to add some "social" to your site? This book covers all aspects of social interfaces.
In interest of full disclosure I must mention that I worked closely with both Erin and Christian at Yahoo. Erin Malone, Matt Leacock and others created the Yahoo! Design Pattern Library. After I joined Erin's team I took over pattern curation for a short while and Erin & I launched the public version of the Yahoo! Design Pattern Library. When I left the pattern curation role Christian Crumlish was wisely brought in to be the pattern curator. While I focused the library on rich interactions (at the start of the Ajax wave), Christian, Matt, Erin and others begin to supply many social patterns. It was from that work that this book is born. It was greatly extended via their Designing Social Interfaces Wiki site (you know make the book on social patterns via social mechanisms!).
If you are designing sites with social features or are just curious about the idioms and patterns in this space then go buy the book!
Also you might want to check out this presentation on the material from this past year's IA Summit.