Dr. Robert C. Worstell, James Breckenridge Jones, Napoleon Hill (2015). “Think and Grow Rich, Updated and Complete - With If You Can Count to Four...”, p.120, Lulu.com
Dr. Robert C. Worstell, Dorothea Brande, Claude M. Bristol, Earl Nightingale, Napoleon Hill (2017). “Mindset StackingTM Inspirational Journal VolumeSS01”, p.168, Lulu.com