The second book our sales coach assigned us as "required reading" was Never Split the Difference: Negotiating As If Your Life Depended On It by Chris Voss and Tahl Raz. I really enjoyed reading it and learned a ton. I had taken negotiations classes before and read other classic books on the subject like Getting to Yes and Getting More (and other related ones like The Art of Asking).
This one took a different perspective and one that was very refreshing because it was based on reality and real-world experiences in very difficult, high-stakes negotiations (with hostage takers). I really enjoyed the riveting stories and accounts of both the successes and failures. I liked the techniques mentioned like the "Late Night FM DJ Voice," mirroring, labeling, asking calibrated questions, and the Ackerman Bargaining Method.
I read it on Kindle and ended up highlighting 242 things (i.e., I learned a lot!). You can read some of those here. I definitely recommend this book to anyone who wants to improve in negotiations of any sort.