In the very last paragraph of Mark Twain's Adventures of Huckleberry Finn, the title character gloomily reckons that it's time "to light out for the Territory ahead of the rest." Tom Sawyer's Aunt Sally is trying to "sivilize" him, and Huck Finn can't stand it--he's been there before.

It's a decision Huck's creator already had made, albeit for somewhat different reasons, a quarter of a century earlier. He wasn't even Mark Twain then, but as Huck might have said, "That ain't no matter." With the Civil War spreading across his native Missouri, twenty-five-year-old Samuel Clemens,...