I don't know if anyone has yet pointed this out, but Sam Clemens
adopted Mark Twain as a pen name for pieces of a humorous nature. And
there already was a long-standing literary tradition in England, the
United States and many other countries for humorists to use pen names.
Charles Dickens' earliest works of fiction, most of a humorist nature,
were published under the pen name of Boz. So were the first two books,
"Sketches by Boz" and "The Pickwick Papers." It wasn't until the third
book, "Oliver Twist," that he relinquished the pen name. By then, his
own real name was as well known and he was tackling increasingly serious
themes with an actual novel (although there remained great affection for
the Inimitable Boz).
    On this side of the pond, Washington Irving presented his highly
satirical "History of New York" as the work of one Diedrich
Knickerbocker, and "The Sketch Book" (inlcuding both "Rip Van Winkle"
and "The Legend of Sleepy Hollow") as the tales of Geoffrey Crayon.
    Long before Twain, Jean-Baptiste Poquelin wrote his comedies and
acted in France under the name of Moliere. During Twain's lifetime,
Anton Chekhov wrote his early sketches for Russian journals under the
pen name of Antosha Chekhonte.
    The custom by Twain's day in America was for humorists to write
under pen names. That certainly was the case with the humorists Twain
knew and admired. So Charles Farrar Browne (a Cleveland Plain Dealer
columnist, no less!) became Artemus Ward. William Wright became Dan De
Quille. Henry Wheeler Shaw became Josh Billings. David Ross Locke became
Petroleum V. Nasby. And Samuel Langhorne Clemens became Mark Twain.
    But while Dickens could toss Boz aside, Twain had no such luck when
he turned to increasingly serious topics. He wanted to publish "Personal
Recollections of Joan of Arc" under his own name (thinking it his most
serious work of literature), but he was so much better known to the
public as Mark Twain, that was the name put on the cover by Harper &
Brothers.
    All best,
        Marcus de Bilgewater Dawidziak