First you have to understand what STYLE of selling you're looking for. A book for one style may be great for me, but you'll think it's weak and ineffective.
Two main distinctions are the "hardcore closer" (traditional) approach versus the "consultative" (more modern) approach. One appeals to an individual while the other does not.
Many people are unaware of anything under than the fast-talking, bombastic style of selling made popular by movies and television, however; it took me 10 years in the professional world to even figure out the right question to ask ("IS there another style of selling?"). Once you understand which style of selling you want to learn or expand on, then you can choose appropriate books or courses.
There is little point in giving SPIN Selling to a Hopkins devotee. And The Challenger Sale will fall on deaf ears to those who are desperate to not be seen as "pushy."
Stephan Schiffman's Cold Calling Techniques is one of the most practical sales books I've read. He has written many others, too.
Dan Pink's To Sell is Human is excellent, too. It takes a non-traditional look at selling and helps understand some of its psychology.
Beyond that, I don't have a lot of specifics. There are many sales methodologies out there, and you should use one. It's not important which one you use -- just that you do adopt one.