This is a question that hits most people who've read the pieces Joseph Mitchell did on Gould. Mitchell makes him so vivid that you really want to know- were they poetic outsider art or garbage? Were they just heaps of nonsense, or were there amazing transcriptions of, say, Ezra Pound talking to e. e. cummings about Mussolini?

Fortunately, Jil Lepore was also seized by the question, and being a good researcher, did her best to track them down. When she started it was a quest, but as she found out more and more about Gould ( and Mitchell) it seems to have become a kind of laborious journey, then a creepy, bad trip....kinda like going for a hike that ends up in a sodden, stinking swamp. She did find a few notebooks. She also found lots and lots of letters, that were filled with sad, revealing things about Joe Gould. It stopped being interesting. In the end, she didn't want to read anymore. She wrote about it for the New Yorker in 2015, which later became a book, Joe Gould's Teeth. I think it's a good example of how sometimes doing history takes you someplace you don't want to be.

https://www.newyorker.com/magazine/2015/07/27/joe-goulds-teeth