''I am a person who would quit authorizing in a minute to go to piloting,'' Mark Twain once remarked. ''I would rather sink a steamboat than eat, any time.'' And in 1882, Twain did just that: he returned to the river of his youth as a mature writer determined to expand seven articles which he had serialized in The Atlantic Monthly in 1875 into the definitive travelogue on the great Mississippi. Although Life on the Mississippi was not commercially successful when first published in May 1883, it is the work that Twain later claimed was the favorite among his books. Twain''s rich portrait of the Mississippi also marks a distinctive transition in the life of the nation, from the boom years preceding the Civil War to the sober times that followed. Yet it is infused with the irreverent humor that was his trademark. ''Mark Twain was the first writer who ever used the American vernacular at the level of art,'' said Bernard de Voto. ''He had a greater effect than any other writer on the evolution of American prose.''