Chris Brogran's Social Media 101. Tons of primers like this have appeared in 2010. But Brogan's towers above the rest for depth and clarity.
Ardath Albee's eMarketing Strategies for the Complex Sale. You won't find a better guide to B-to-B marketing success in the age of social media.
David Meerman Scott's Real-Time Marketing and PR. I cannot say enough praiseworthy things about Scott's 200-page treatise. It's mandatory.
Mitch Myerson's Success Secrets of the Social Media Marketing Superstars. Everything you ever wanted to know about social media marketing (but were afraid to ask).
David Meerman Scott and Brian Halligan's Marketing Lessons from the Grateful Dead. Informative, inspiring and a delight to read.
