Various historians have painted him as either a dumb hick who didn't realize what he was getting into, a patsy who was set up by the investigators to be a highly visible fall guy, or one of the core group of match fixers who received one of the highest payouts.
Jackson himself originally confessed to involvement, then recanted, and maintained his innocence throughout his life.
What's the truth (or as close as we can get to it)?
Jackson knew about the plot, kept $5000 (delivered to his hotel room after the last game of the series by pitcher Lefty Williams), and admitted in grand jury testimony that he could have tried harder on a couple of fly balls.
But he told Williams that he didn't want the money, and that he was going to tell Comiskey everything he knew about the fix. The next day he did try to see Comiskey, who refused to meet with him, and Harry Grabiner, Comiskey's secretary, "told me to take the money and go to my home in Savannah. He told me if anything further was to be done, he or Comiskey would write me about it." And he hit .375 in the series with twelve hits (a WS record at the time) and the series' only home run.
From there on, it's a matter of interpretation.
Source: Harvey Frommer, Shoeless Joe and Ragtime Baseball. Taylor Publishing, 1992.
I have a related question.
This Jonathan Coulton song mentions Shoeless Joe Jackson. Which parts of the song are accurate and which ones are embellishments?