Napolean Hill is the FATHER/FOUNDER of the entire success/motivation field, and especially of the systematic study and application of principles of success.
Everyone since then -- Ziglar, Tracey, Robbins, Hopkins, and many, many more -- all owe it all to Hill.
Some of his writing/speech is a little dated (because it's literally not modern English), but it's understandable and in some ways that makes it all the more interesting.
One thing is for certain - the man KNOWS what he talked about, and he was determined to pass it on to you.
Everything Hill wrote/recorded is worth reading/listening to ... over and over again, until it fully sinks in!
NOTE: This is NOT actually a book on selling. It's a book on personal success.
I do highly recommend it to salespeople, just as I do to anyone else who actually wants to succeed in life and is willing to grab life by the horns and go for it.