Mark Twain (2014). “Mark Twain on Common Sense: Timeless Advice and Words of Wisdom from America's Most-Revered Humorist”, p.53, Skyhorse Publishing, Inc.
Mark Twain, William Dean Howells (1960). “Mark Twain-Howells Letters: The Correspondence of Samuel L. Clemens and William D. Howells, 1872-1910”, Belknap Press