By far one of the best volumes devoted solely to _Huckleberry Finn_ is:
Doyne, Victor. _Writing Huck Finn._ U. of Penna. Press, 1991.
You also may want to consult:

_Mark Twain: The Fate of Humor_ by James Cox
_Mr. Clemens and Mark Twain_ by Justin Kaplan
_Mark Twain at Work_ by Bernard DeVoto
_Mark Twain: Social Critic_ by Philip Foner
_Was Huck Black?_ By Shelley F. Fishkin
_Comedic Pathos: Black Humor in Twain's Fiction_ by Patricia M. Mandia

The above is just a miniscule portion of the very tip of the iceberg of
Twain
scholarship. The texts and articles on Huck Finn could go on and on forever.

Marcus W. Koechig