This book is probably one of the biggest books ever written about anything. The main subject of course is money and success. The book is divided in "small" 16 lessons each one talks about a particular human trait you must master to be a successful man.
The book tells that you must be willing to be higher than yourself and to do things in a way you never did before like doing more than you are paid for, to be always the first person taking action and not to be jealous. This lessons teaches specially that if you work as little as you can just to get paid your ordinary salary your services demand will cease because you made yourself useless. Work all you can and give all you can.
In another lesson Napoleon Hill teaches us that our mind is not only what we can think. Some things are much more powerful and when that enters our mind nothing can make it not happening. For example, in France one man was sentenced to death by decapitation. They made a small experiment on that man. Instead if a guillotine they placed a fake guillotine that would just simulate a cutting razor. Next they warmed some water and they run the experience. The simulated the sound of the guillotine, then made the warm water run over his neck to simulate blood and waited for the results. After just eight minutes the man was dead. How was that possible? He thought that his neck was cut and all his body accepted that as well so his body dies because thinks he doesn't have his head anymore. Lesson here: Our thoughts make our results real. Think success and you will be successful!
In my favorite lesson Napoleon Hill talks about how hard we should work. He gives the example of one Greek man sentenced to hard work in the prison. When the chariot race event took place on the Colosseum the recruiters went to that prison and chosen that man to race. As you know you need a lot of strength to ride all that crazy horses in the heat of such dangerous race! That man made it and his prize was Freedom! The men there asked him: How did you get such strong arms?" The man answered: "AT THE PRISON" Lesson here: You need to make yourself work just as hard as you can!
Finally Napoleon Hill teaches us Henry Ford greatest lesson. Ford used to say that he would give costumers the best car at the cheapest price he could, paying the best wages he could. The result was getting the best qualified workers that did not need too much checking because they were happy to work at such good salaries and Henry Ford could sell the best cars in the world at the cheapest price!