7 Steps - Sales: Question for the masses

In "7 Steps" author Benjamin Suarez stresses sales as the key reason for business success or failure. He also states on page 1-32 that "Unless you have been exposed to hard-to-find knowledge about how to sell, you won't succeed."

So Gordon, Dien & Co., besides going door-to-door selling Encyclopedias - what are the best sources of slaes knowledge out there?
There are quite a few books, tapes and seminars but which ones do you highly recommend?