Dr. Robert C. Worstell, Dorothea Brande, Eark Nightingale, Claude M. Bristol, Napoleon Hill (2017). “Mindset StackingTM Inspirational Journal VolumeSS02”, p.150, Lulu.com
Napoleon Hill (1960). “The Law of Success in Sixteen Lessons: Teaching, for the First Time in the History of the World, the True Philosophy Upon which All Personal Success is Built”