Albert Schweitzer, William Larimer Mellon (1996). “Brothers in Spirit: The Correspondence of Albert Schweitzer and William Larimer Mellon, Jr”, p.18, Syracuse University Press
Dale Carnegie, J. Oliver Crom, Michael A. Crom (2003). “The Sales Advantage: How to Get It, Keep It, and Sell More Than Ever”, p.84, Simon and Schuster
Dale Carnegie, J. Oliver Crom, Michael A. Crom (2003). “The Sales Advantage: How to Get It, Keep It, and Sell More Than Ever”, p.193, Simon and Schuster