Steinbeck is also really worthwhile. Not as consistent as Hemmingway, overall, but his subject matter was more varied.
I'd like to recommend Faulkner right off the bat, but he is an incredibly talented, difficult writer, and I have a feeling that one should have a fairly nice understanding of 19th century American history before getting into his classics. Faulkner is, by a very very wide margin, the best American writer in the country's history.