It's very rare for me to talk about a non-fiction book that doesn't involve science. It's even more rare for me to talk about a "social media expert," as most of the people who self-annoint themselves with this title are douchebags who haven't done anything other than blog about someone else's accomplishments, then try to charge companies consulting fees to share their "expertise" or, even worse, their "guru-ness."
So if I'm recommending a non-fiction book by a social media expert, I think that speaks for itself, TRUST AGENTS by Chris Brogan and Julien Smith is one I've been waiting to check out, solely based on the two guys that wrote it. Brogan is a super nice guy, has a blog with solid information, and has gone out of his way to help promote my books. Julien Smith I know from way back in the early days of podcasting, circa 2005. He is smart, wicked-cool, and proves his internet knowledge based on hard sales to end customers.
If you already use social media to build customer relationships, most of this will probably be common sense to you. However, if you're looking to use social media to build your business, and do it in a non-douchebaggy way, these cats know their shit. They prove their techniques via large numbers of readers and via sales