I am taking a negotiation class right now and we are using the three books listed below. The professor is really great, and the books are not boring college textbooks - they're really engaging and I'd read them on my own if I wasn't taking the course, too.

"Yes!: 50 Scientifically Proven Ways to Be Persuasive" by Goldstein, Margin, & Cialdini is a great book on influence and persuasion.

Cialdini is a great resource for influence/negotiation in general, so look for other articles/books written by him.

"Predictably Irrational, Revised and Expanded Edition: The Hidden Forces That Shape Our Decisions" by Ariely is a great book on behavioral economics

"Negotiation" by Lewicki, Saunders, and Barry