#OTD in 1863.
Samuel Langhorne Clemens, in signing a humorous letter to the Territorial Enterprise newspaper in Virginia City, Nevada, first uses the pen name Mark Twain.
Twain first used his pen name, when he wrote a humorous travel account titled "Letter From Carson – re: Joe Goodman; party at Gov. Johnson's; music" and signed it "Mark Twain".
https://en.wikipedia.org/wiki/Mark_Twain_at_the_Territorial_Enterprise
Books by Mark Twain at PG:
https://www.gutenberg.org/ebooks/author/53