Napoleon Hill
1883-1970
Anyone today that speaks on a success philosophy will at some point, at the very least, quote Napoleon Hill. It's safe to suggest that the philosophy of success he described has brought more wealth and success to more people around the world than any other writer of the last hundred years or so.
Commissioned by Andrew Carnegie, Napoleon spent a lifetime studying several hundred of the most successful people of his time. He was close friends with the likes of Henry Ford, Thomas Edison, and hundreds more of similar calibre. He worked closely with previous presidents of the USA, implimenting plans to lift the country in various ways. His most famous book, Think and Grow Rich (audio book linked below) has sold over 100 million copies and has more recently been turned into a film.
Even today, long after his death, (despite the attempts of some) his legacy continues to grow increasingly stronger.