These days it's life of the author plus 70 years, or 100 years depending upon whether or not the work was created by individuals or a corporation "for hire". Got to keep "Steamboat Willie" under copyright forever.
All of Twain's works are public domain now. Most of Mark Twain's works are available for free through Project Gutenberg. I would imagine the copyright issues at question have to do with the play itself, which, if it was written after 1925 or so, would still be under copyright, not the book that it is based upon.
Thanks fer the info.
An aside:
Where does one go to find out if a specific writing is in public domain?
:)