I was interested in learning about negotiation a few months ago, so I bought all the best-reviewed books on Amazon. Here's the ones I especially liked: The Power of Nice (Shapiro), Bargaining for Advantage (Shell), 3D Negotiation (Lax), Negotiation Genius (Malhotra)
thanks!
'how to win friends and influence people' isn't specifically about negotiation, but its a classic and the principles found in there can easily be applied.
The Secret of Power Persuasion is an old favorite.