Groundswell is a great book for an introduction to social media, some strategies, and some case studies.
The next step up, a book that I loved, is The Whuffie Factor by Tara Hunt (a lot of her philosophy and case studies can even be read on her blog). This gives loads of really fantastic examples of how brands embraced social media and used it to turn their brand around. Tara is a great writer and the book is an easy, enjoyable read. I would highly recommend this as a Spring Break read for anyone interested in learning more and really seeing how brands make the most of social media and the new consumer environment. Tara represents a outlines the idea of "whuffie" (social capital) and how important it is to Web 2.0, which is something that I think is crucial to a great online interaction with consumers.