You might find books on social psychology helpful. At one point in his career, my ex was a military recruiter. They get world class sales training. He borrowed my textbook from the social psych class I had taken and I didn't see it again until his tour of duty as a recruiter was over. I will also recommend "Getting to yes" which is research based and a quick read and "The mind and heart of the negotiator", which is also research based but meatier. I believe there is a free version of the latter available online. These were both required texts for my class on conflict management and negotiation.
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