It seems that, in passing, some books are referred to as "an American classic," or "one of the great American classics." This seems like it's a whole subsection of what counts as "classic literature."
Oddly, while infrequently, certain books are debated, or people discuss which new book is going to be canonized as the next Great American Book, most of the books that we describe as "classics" are ones that nobody would bother to refute. Everyone seems to agree, or assume, that they are. Stuff like The Adventures of Huckleberry Finn, or The Jungle, or For Whom the Bell Tolls, or 1984, or Of Mice and Men, or The Great Gatsby...
Then we have the American classic authors, some of whom appear in that list: Mark Twain, Hemingway, Steinbeck, Fitzgerald...
Nobody disputes that these are "classics". But, how did they get labeled that way? Does somebody decide which books are classics and which aren't? Is there a process for canonization of the body of classical literature that I'm just not aware of?
If there isn't, then how did this terminology come about, and why is it so consistent everywhere you look in America?