Mark Twain was a favorite with "Bonanza," appearing in three episodes over
the course of the show.  At least there was a natural tie-in:  the Ponderosa
was supposedly near Virginia City, and it was set roughly during the time
Twain was in the area.  The one I remember best is called "Enter Mark
Twain," which was the fifth episode in the series, from 1959.  Here's the
imdb.com listing, with Howard Duff playing Mark Twain.

http://www.imdb.com/title/tt0529553/

And a bonus:  it will be airing tomorrow, January 11, at 4:00 pm on TVLand!
I may skip class...