The Greatest Salesman in the World is a novel about how one can become the greatest salesman in the world. Though the story is interesting, I don't think I learned much from it at all.
The book espouses some principles that will make one a better salesman. Those principles seemed fine enough to me, and could apply to anyone in life, not just salesmen. But they are principles that you already know. Things like: "appreciate what you have" and "love yourself and others". These are excellent things to go by, but I'm not sure they add any value for people who are already happy.
So for me this was more like a self-help motivational book. It may be useful for someone who needs a kick in the pants, but I don't think it's useful for actually learning any techniques. There is a religious bent to the story too, so if you're the type you may appreciate it for that reason.