If you're going to prosper in the coming years, your sales team needs customers, prospects, vendors, and peers to say "yes" more often than "no." Getting to "yes" requires a substantial degree of persuasion and influence skills. Contrary to popular belief, persuasion and influence take much more than just a ready smile, a big handshake, and an outgoing personality.
Robert Cialdini published a book in 1993 called, Influence: The Psychology of Persuasion, which discusses, investigates, and proves how influence works. This book is a must-read for your entire revenue team in sales, marketing, or general business development.