The only book that really matters.
https://www.amazon.com/Soft-Selling-Hard-World-Persuasion/dp/0762404019
Just remember that if you are not there to actually help people, (other than yourself) then you are not relevant.
So in order to understand the customer and to know how to apply your product to make THEIR life better, you have to ask questions and you have to listen...and really understand. Put yourself in their shoes.
Having heart and being honest and really wanting to help people is what sales is all about.