Well, I suppose it's a "classic" if you're into New Deal socialist propaganda fine-tuned by Hollywood members of the Communist Internationale. I'll give Steinbeck's politics the benefit of the doubt but Hollywood Communists turned his book into a "classic" piece of collectivist crap.
If you have any interest in learning what a Stalinist stooge FDR was, read Diana West's "American Betrayal: The Secret Assault on Our Nation's Character." It's life-altering.
You do know that Daryl Zanuck and John Ford were conservatives right?