Gary,
What I have isn't big enough to be a book...more like a chapter out of a book. It's printed in "The Civil War Archive," a book edited by Henry Steele Commager. The full title listed in the book is "The Private History of a Campaign That Failed." About it, Commager says:
...he (Twain) joined a volunteer company of a dozen or so, which, according to his own story, was ready to fight on either side. "Out West," he wrote, " there was a good deal of confusion in men's minds during the first months of the great trouble...it was hard for us to get our bearings." His campaign came to an inglorious end when he sprained an ankle falling out of a barn hayloft. One of the "Marion Rangers," Absalom Grimes, later became a famous Confederate scout. As Mark Twain himself says, this is not an unfair picture of what went on in the border states during the early months of the war."
Twain remarks about the great trouble: "I was piloting on the Mississippi when the news came that South Carolina had gone out of the Union on 20th December, 1860. My pilot-mate was a New Yorker. He was strong for the Union; so was I. But he would not listen to me with any patience; my loyalty was smirched, to his eye, because my father had owned slaves. I said, in palliation of this dark fact, that I had heard my father say, some years before he died, that slavery was a great wrong, and that he would free the solitary Negro he then owned if he could think it right to give away the property of the family when he was so straitened in means. My mate retorted that a mere impulse was nothing -- anybody could pretend to a good impulse; and went on decrying my Unionism and libeling my ancestry. A month later the secession atmosphere had considerably thickened on the Lower Mississippi, and I became a rebel; so did he. We were together in New Orleans, the 26th of January, when Louisiana went out of the Union. He did his full share of the rebel shouting, but was bittely opposed to letting me do mine. He said that I came of bad stock--of a father who had been willing to set slaves free. In the following summer he was piloting a Federal gun-boat and shouting for the Union again, and I was in the Confederate army. I held his note for some borrowed money. He was one of the most upright men I ever knew; but he repudiated that note without hesitation, because I was a rebel, and the son of a man who owned slaves."
Twain goes on to tell how he and other young men from Missouri formed the "Marion Rangers," and of their well-intentioned but decidedly clumsy attempts to be soldiers.