Edgar M. Branch has written about the Pilots' Association in:

Title: Mark Twain and the starchy boys /
Author(s): Branch, Edgar Marquess, 1913-
Publication: Elmira, NY : Elmira College Center for Mark Twain Studies at
Quarry Farm,
Year: 1992
Description: iv, 97 p. ; 23 cm.

I believe that at one time Branch was also working on providing
annotation for a University of California Press edition of _Life
on the Mississippi_,  I'm not sure what the status of that publication
is now.

Another source for steamboat terminology not mentioned in previous
responses to the earlier question is:

Title: A Mark Twain lexicon,
Author(s): Ramsay, Robert L. 1880-1953.  (Robert Lee),; Emberson, Frances
Guthrie,; 1912- ; joint author.
Publication: New York, Russell & Russell,
Year: 1963


Barb