The Master Key System by Charles F. Haanel

The Master Key System is a personal development book by Charles F. Haanel that was published in 1912. Though originally a 24 week correspondence course, it was published in book form in 1917. Along with "The Science of Getting Rich", by Wallace D. Wattles, the Master Key System is the source of Rhonda Byrne's inspiration for the book and the film "The Secret" (2006).