I have done thousands of negotiations (for vastly different things) and have been paid to do so. [1] I have never read a single book on negotiations. I can always tell when someone else has. (Read that again). [2] I could most likely write a book myself on negotiations and my experiences. The problem is it's a seat of the pants thing. It's nearly impossible to be able to say what to do unless you are in the exact situation and reading all the signals. Also it's creative at it's core so if you are a narrow type thinker you are most likely not going to be good at it no matter what (very generally). And of course what you are negotiating for and in particular who you are dealing with and their motivations are super important to the process. That said I am sure a book can help with an adversary that is not particular difficult and of course better than nothing. But like with most other things it is in no way even close to a substitute for actual experience over time and the right base attributes personally. Some people just aren't perceptive enough of others emotions and reactions to be good at it. No way around it. Also their own emotions and the ability to control them.
[1] The largest fee I made (recently) was close to $200k and I've been paid as little as $500 (or have done it for free in some cases).
[2] Because their delivery and actions appear canned and predictably fit another pattern that I have seen in the exact same way over time.