I am not a die-hard Jeopardy fan, but I've watched it a few times. How could I not? It's part of American lexicon. And I was pleased that I knew a few of the answers. ( Yet, I couldn't twist my brain around to phrase the answer as a question.) But the statement that caught my eye in the NY Times article of April 23, 2019, is Holzhauer's attribution about where his knowledge comes from:
"As for the sources of his knowledge, Holzhauer has said that an underrated strategy is reading children’s books, which he said are more effective than adult books because they cater to readers who might not be naturally interested in the subject matter."
I am presuming that he is referring to children's nonfiction because he is mentioning "subject matter," i.e. "content." He is not referring to "textbooks" or school books or even school. He's a reader who long ago discovered what we, who write children's nonfiction, hope that the education community and maybe even the patrons of school and public libraries would also discover. If you want to learn about something new, read a kid's book or two on the subject. You'll soon learn who the best nonfiction authors are. They are the ones who treat the reader as an intelligent person (not using over-simplified language or "dumbing down" a subject) but assume that their reader has little or no prior knowledge on the subject. We write for the uninitiated on content that captivates us as authors so that our writing will also captivate our readers. We use a lot of the techniques of the best fiction authors and have invented a few tricks of our own.
A study in 1988 showed that good writing is memorable. That concept has paid off very well for James Holzhauer. Maybe those who marvel at the depth of his knowledge will be inspired to follow Holzhauer's course of study. He read for pleasure about the real world, he followed his interests when he read, and he studied Jeopardy from childhood to master the moves of the game. He learned that he could play the game and play it well before he became a contestant.
I'm betting that he's still reading children's nonfiction.