Think and Grow Rich – 70 million
Author Napoleon Hill is sometimes considered the father of the self-help movement in the U.S. He studied successful and wealthy people and then created a formula for achieving personal success. This, his most popular book, was released in 1937 during the Great Depression. It distilled his formula into a philosophical system for success that apparently resonated with people struggling to get by. It stayed on best-seller lists for many years, and spawned an entire industry of books about how to be a better person.