It's probably out of date now but it inspired me to become an entrepreneur in 1994.
Also this is the best book I've ever read on negotiating by Chester Karass https://www.karrass.com/dr-chester-karrass
For me, the key to negotiation is to try to get to a fair deal for everyone, not a deal in which someone wins and someone loses. Ask everyone involved what they want out of the deal, what they think would be a fair outcome. Where there is a gap between what the parties want, ask each party how far they are willing to move.
Don't do deals that are unbalanced or unfair because the deal falls apart a bit further down the track leaving people feeling annoyed, angry, let down, ripped off.
Remember if the parties are too far apart on what they want then probably there is no deal to be done. Just let it go - no deal, move on and do business somewhere else.