People that stacked How to Win Friends and Influence People
A classic with lots of good examples on the psychology of persuasion. Goes into the nitty gritty details, and makes you think about your approach to communicating with people in general (not only in the context of sales).
Growing up I was pretty introverted, I wasn't really good at talking to people or even making friends. This book pointed me in the right direction and got me to start getting better socially.