Are you sure about that? My dad grew up in the Depression. He sat next to Okies in school in Long Beach. He also had a doctorate in history.

So I asked him about TGoW. He said it was an exaggeration, as was most of Steinbeck's work.

As for TJ, Sinclair could have done it as an actual history book, i.e. putting down the results of his rigorous research. But he didn't, and so he could make up whatever he needed. The entire book was a pitch for socialism.

Unfortunately, in time both will become definitive historical reference sources, if they haven't already.

Before the collapse of Google Books, when for a brief, glorious moment Google came closest to achieving the dream of making accessible all the world's knowledge, I could much more easily trace back historical "facts" to a single popular book in whatever field, but then keep going back through older material and often find contradictory accounts and narratives.

History is messy. There's of course what actually happened, but almost immediately narratives start to branch. Then at some point, years, decades, or maybe centuries later, narratives converge back to a single "truth", typically based upon a single, popular author's gloss. There can be some subsequent divergence and reconvergence, but that first convergence really seems to be nearly as defining if not more defining than the actual event(s).