"Best You Ever: 365 Ways to be Richer, Happier, Thinner, Smarter, Younger, Sexier, and More Relaxed - Each and Every Day". Book by Rebecca Swanner (p. 324), October 18, 2010.
Theodore Roosevelt (2015). “Theodore Roosevelt on Bravery: Lessons from the Most Courageous Leader of the Twentieth Century”, p.5, Skyhorse Publishing, Inc.