“While the overall goal of marketing remains the same, the advent of social media has changed the role of the marketer from a broadcaster pushing out messaging and materials to an aggregator who brings together content, enables collaboration, and builds and participates in communities.”

Paul Gillin, author of “The New Influencers,” suggests four books on social media. He gives a short and sweet description of each.
Everything Is Miscellaneous by David Weinberger
The New Rules Of Marketing And PR by David Meerman Scott
Marketing to the Social Web by Larry Weber
Join the Conversation by Joseph Jaffe