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