Getting along with people
Books commenters say helped with conversations, empathy and relationships.
Books several people mention
Ranked by how many different people say it helped. Small numbers, so the order is a hint.

Classic advice on getting along with people, making them like you, and persuading them.
Dale's book helped me go from virtually no friends to having many treasured relationships in my life, across interests and divides that I never would've even bothered to have explore if not for that book.

A method for speaking and listening with empathy to resolve conflict at home and work.
NVC gave me a framework to communicate in a healthier way. This wasn't just something that helped at work, it had an enormous effect on my personal life as well and I credit it to having strong relationships now.

Techniques for staying calm and getting results in high-stakes, emotionally charged conversations at work or home.
The techniques I've picked up have helped me build stronger relationships with my spouse, my children, my customers, my employees, and my peers.

A journalist's account of joining the pickup artist community and learning to seduce women.
Read "The Game" by Neil Strauss. Changed my life. Primarily, I realized my model for interacting with women was based on familial roles e.g. empathy, sharing emotions etc.

Negotiation tactics from a former FBI hostage negotiator, usable at work and home.
The book changed the way I handle communication. There are actionable practical tips and tools for communication and it shaped how I think about a hard phone call, how to prepare for a meeting

An argument for the strengths of introverts and how society undervalues them.
It was a paradigm shifting book for me, made me understand some people in my life in profoundly new ways, and helped me discover stuff about my own personality.

An investigation into the roots of morality and why people divide over politics and religion.
After reading the book, my politics are still left of center (but definitely right of Sanders), but I think I understand and appreciate the politics of my right leaning family and friends.

A step-by-step approach to handling tough talks with less stress and better outcomes.
I used to believe that I was great at communication and bad at negotiation, but the opposite was true; DC has literally changed my life more than any other non-fiction book

An explanation of what charisma is and how anyone can learn to build it.
I went from being extremely uncomfortable in pretty much any social situation, to being able to hold a conversation with almost anyone, including strangers on the street, to then people wanting to be with me and to lead them

Guidance on knowing when to say yes or no and taking control of your life.
The book Boundaries: When to Say Yes, How to Say No to Take Control of Your Life by Henry Cloud and John Townsend is what helped me work through this.