Charles F. Haanel (1866-1949) used the concepts and methods found in The Master Key System to create a successful business career in St. Louis, Missouri, where he eventually founded one of the largest conglomerates of his time. He wrote several bestselling books focusing on his business technique. Besides The Master Key System, he also wrote Mental Chemistry and The New Psychology. Known as the "father of personal development," Haanel's commonsense wisdom has revolutionized conventional thought for almost one hundred years.