I've just started Yes! 50 Scientifically Proven Ways to Be Persuasive by Noah J. Goldstein, Steve J. Martin and Robert B. Cialdini, and can already give it my highest recommendation.  It offers fifty short lessons (2-4 pages each) on persuasiveness, along with the empirical evidence to back them.

One quick lesson from the first chapter in the book:  Simply by changing an infomercial's call to action from "Operators are waiting, please call now," to, "If operators are busy, please call again," resulted in a huge increase in products purchased. 

Why?  Instead of people imagining a room full of operators waiting by silent telephones, infomercial viewers imagined those same operators going from call to call without a break, and assumed "if the phone lines are busy, then other people like me who are also watching this infomercial are calling, too."

Very interesting stuff.  A highly recommended book!