Steinbeck by far. The Grapes of Wrath is frighteningly relevant now even almost 100 years later..

I think that steinbeck is probably the correct answer. the iliad and the odyssey are not just "action epics", they are parable-ish in what they communicate to readers (or listeners) of their society. For example, it's not a coincidence that odysseus sleeps around the aegean sea while penelope chases suitors off at home.

Twain is a pretty good second choice, but Steinbeck has a pretty good lock on parables of American aspirations and its society both good and bad.

Steinbeck also wrote "the pearl" and of course "of mice and men"