It's not so clear cut here; Steinbeck submitted it for publishing, got rejected, then seemingly spent years improving his art and produced his major works.
I certainly don't blame him for not wanting to throw a werewolf murder mystery out there after being recognized for what he must consider his "real" work. But it was commonplace to destroy unwanted manuscripts or simply to say, don't publish it, or destroy it when I die, etc, and he didn't. To me it seems like license to print, as long as the context is explained in a good introduction. Scholars of the American novel will go nuts for it.