At the end of a long and prolific career with the pen, America's favorite humorist grew reflective about his craft, yet kept his tongue firmly planted in his cheek: "I have always been able to gain my living without doing any work; for the writing of books and magazine matter was always play, not work. I enjoyed it; it was merely billiards to me." Thus could Samuel Clemens, writing in Mark Twain's Autobiography, sum up a half century of authorship and the penning of such classics as The Adventures of Tom Sawyer, The Adventures of Huckleberry Finn, and Life on the Mississippi as "Billiards." But as with so much else about Clemens, what was meant for public consumption was a very different matter from what went on in private.

Clemens's pseudonym, Mark Twain, is perhaps the most appropriate one in literature to date. Adapted from riverboat parlance to...