Hal, I don't know if it is quite what you're looking for, but I've found
Thomas Buchanan's _Black Life on the Mississippi:  Slaves, Free Blacks and
the Western Steamboat World_ (Chapel Hill: U of NCP 2004) to be a useful
volume.  He opens with a contrast between the world Twain envisioned and
experienced on the Mississippi and very different world seen and experienced
by William Wells Brown, who also worked on a steamboat--and goes on from
there.  Solidly researched and well written.

Sharon