Amby Burfoot (2000). “The Runner's Guide to the Meaning of Life: What 35 Years of Running Have Taught Me About Winning, Losing, Happiness, Humility, and the Human Heart”, p.24, Rodale
Ken Schwaber, Jeff Sutherland (2012). “Software in 30 Days: How Agile Managers Beat the Odds, Delight Their Customers, And Leave Competitors In the Dust”, p.51, John Wiley & Sons