Elbert Hubbard (2009). “Love, Life & Work: Being a Book of Opinions Reasonably Good-Natured Concerning How to Attain the Highest Happiness for One's Self with the Least Possible Harm to Others”, p.98, The Floating Press
Stephen R. Covey (1994). “Daily Reflections for Highly Effective People: Living THE SEVEN HABITS OF HIGHLY SUCCESSFUL PEOPLE Every Day”, p.132, Simon and Schuster