I thought I'd mention three books I've read about negotiation, each of which takes a different approach:
Herb Cohen's You Can Negotiate Anything discusses what I'd call the theory of negotiation, the important principles involved: time, power, information. He mixes humorous examples (an insurance adjustor who gave him hundreds of dollars just because he didn't say anything in response to the first X offers, for example) with ideas. He talks about the worst person to negotiate for (yourself) and about being willing to coming to an agreement by discussing considerations other than money. Some of the examples are a little dated, but overall I'd say this is a timeless overview anyone involved in business should read at least once. (And if you ever buy or sell anything, including your labor, you are involved in business!)
Leigh Steinberg's book Winning with Integrity (written with or perhaps by Michael D'Orso) contains more and less practical advice. If Cohen covers the principles, Steinberg covers numerous specific rules of thumb -- applications. Steinberg is a sports attorney who, if the stories in this book are the whole picture, always manages to negotiate higher contracts for the players he represents than any that have gone before. I don't agree with everything he says; for example, for him, the goal of negotiation is to get the best deal possible, but I'm satisfied with a good enough deal. (Note that "good enough" may be quite a high bar, as discussed in The Paradox of Choice, another good read.) Still, if you're selling anything, it's worth considering his ideas to see if there are any that might help you come to an agreement (read: sale) more easily.
Robert Cialdini's Influence: The Psychology of Persuasion discusses the touchy-feely factors that can lead to agreements. It's both entertaining and horrifying when you realize how irrational the decisions we make are. The edition I read included ideas about how to avoid these techniques being used on you by salesmen, so it has a slight pro-customer bias, but mostly I found it to be a make-you-think read about why and how we make decisions about who to trust and what to do. Even tactics that are obviously fake (sitcom laugh tracks are cited as an example) can have a substantial influence on what you're willing to do or agree to. Definitely worth reading for anyone who makes decisions (and that's everyone).