"properly understood" is the whole point. That's the hard problem they are trying to solve.
Given that Jeopardy focuses a lot on literature, I'd also throw in gutenburg project books. And probably a newspaper archive going way back, like the NYT.
Beyond that, you want software/heuristics. You might find far more data helpful to initially create that software, but once it's created, the reference material to have at hand can come from a small set of sources.