When I wrote my now severely out-of-date dissertation (now in the mark Twain
Forum archives), I spent one chapter talking about the dearth of
publications on Twain's western years.  I pointed out much of Twain's
material from that time was lost for various reasons, and that most
scholarship is keyed to Twain's more mature work, for obvious reasons.
Anyway, you can pick up an overview and bibliography  of what was published
up till 1990 on this topic in MARK TWAIN: CRADLE SKEPTIC somewhere in the
Forum contents.  To me then, and to me still, the Dean of this area of
scholarship is Edgar Branch whose best work is in his commentary for various
volumes from the Mark Twain Papers (EARLY TALES AND SKETCHES in particular.)

Wes Britton