Bob,
You may be interested in _The Further Adventures of Huckleberry
Finn_ by Greg Matthews (Crown Publishers, 1983).
Also see the review of _Huck Finn and Tom Sawyer Collaboration;
The Sequel to: Adventures of Huckleberry Finn_ by Twain and
Stephen Stewart (New Mill Publishing, 2001). Reviewed on the Mark
Twain Forum, the review is online at:
http://www.twainweb.net/reviews/StephenStewart.html
Barb