I tend to think backwards rather than forward, so I can't tell you what the
most recent writings about Twain might be where he's treated as a suthrun
writer, but I can tell you the earliest I have seen (in a book)--

Alexander De Menil's THE LITERATURE OF THE LOUISIANA TERRITORY (St. Louis,
1904). This is a survey of more than fifty writers connected in some way to
Louisiana --Kate Chopin, Mollie Moore Davis, Edgar Howe, and of course
Cable, Audubon, Flint, Benton, and a host of minor authors nobody would
recognize. Curiously, he leaves out Hearn, Nye, Garland, and Kate Field.
Twain gets a few pages, but De Menil puzzles over how Twain could be so
popular, Twain being such a crude, unoriginal, commonplace writer, and
concludes it might be because he amuses, but that his fame will not last. He
also gets some facts of Twain's life wrong, and includes him because of
Twain's connection with St Louis and steamboating (remember this is the La
Terr., not the state) rather than a distinct southern influence on his
writings.

Kevin Mac Donnell
Austin TX