“I’ve struck it!” Mark Twain wrote in a 1904 letter to a friend. “And I will give it away—to you. You will never know how much enjoyment you have lost until you get to dictating your autobiography.” Thus, after dozens of false starts and hundreds of pages, Twain embarked on his “Final (and Right) Plan” for telling the story of his life. His innovative notion—to “talk only about the thing which interests you for the moment”—meant...