Mark Twain, Charles Neider (1987). “The outrageous Mark Twain: some lesser-known but extraordinary works : with "Reflections on religion" now in book form for the first time”, Doubleday Books
Mark Twain, William Dean Howells (1960). “Mark Twain-Howells Letters: The Correspondence of Samuel L. Clemens and William D. Howells, 1872-1910”, Belknap Press