David McCullough (2011). “David McCullough Library E-book Box Set: 1776, Brave Companions, The Great Bridge, John Adams, The Johnstown Flood, Mornings on Horseback, Path Between the Seas, Truman, The Course of Human Events”, p.1000, Simon and Schuster
Harry Emerson Fosdick (2008). “Answers to Real Problems: Harry Emerson Fosdick Speaks to Our Time: Selected Sermons of Harry Emerson Fosdick”, p.160, Wipf and Stock Publishers