"How to Win Friends and Influence People" by Dale Carnegie [1] has some very interesting concepts, despite its terrible name, IMO. There are many takeaways but a couple are that people generally like hearing their name and enjoy talking about themselves.
Then there are two very similar, but IMO, complementary books, "Difficult Conversations" by Douglas Stone, Bruce Patton, and Sheila Heen [2], and "Crucial Conversations" by Joseph Grenny, Ron McMillan, Al Switzler, and Kerry Patterson [3] which helped me learn to embrace being genuinely curious and accept + explain different, sometimes conflicting, emotions. It was especially helpful to help me deal with people who were very confrontational and tried to create conflict, and really difficult situations while conversing.
There have been some more books I've read, which I don't recall the titles for and don't even have notes for, but these 3 and "Never Split the Difference" you've mentioned were certainly the most interesting and helpful in this "area", to me.
[1]: https://www.amazon.com/How-Win-Friends-Influence-People/dp/0...
[2]: https://www.amazon.com/Difficult-Conversations-Discuss-What-...
[3]: https://www.amazon.com/Crucial-Conversations-Talking-Stakes-...
I hope that helps!