The Mississippi River meant a lot more to Twain and his contemporary readers
than a boundary between innocence and maturity, and it more often meant a
lot less. To begin with a beginning, you might wish to pick up Twain's own
LIFE ON THE MISSISSIPPI where you can find a multitude of clues about what
the river meant to Twain (and to Sam Clemens as well).
If you want to try the ol' English 101 compare/contrast route, I'd shy away
from "river books" written after HUCK FINN. They show Twain's influence, but
they give not a clue to what the river meant to Twain, these books being
years off in the future from 1885.
How about "river books" that Twain might have read? He certainly read BEYOND
THE MISSISSIPPI (1867). He might have read Thoreau's A WEEK ON THE CONCORD
AND MERRIMACK RIVERS (1849). He owned two volumes of Thoreau's works from
the Riverside Edition, and it's reasonable to assume he could have owned the
entire edition. He owned a copy of Merrick's great memoir/history of
Mississippi steamboating. Browse Alan Gribben's MARK TWAIN'S LIBRARY, A
RECONSTRUCTION for other river books.
Always remember that even Freud once admitted that a cigar is sometimes just
a cigar, and Freud saw Twain on stage with a cigar (and very likely in
person, though it's not documented). Have fun, explore, light out for the
territory, but remember that sometimes a river is just a river.
Kevin Mac Donnell