The cop-out answer is, “Anything you’re interested in, as long as it’s legal.” So, all of Project Gutenberg, Shakespeare’s plays and sonnets, The Bible, Tao Te Ching (it’s short), Mahabarata (it’s long). My wife is studying As You Like It, so I could put that up.
You get the idea. Your notes would be great, but you can’t post those books (without permission).
It just seems to me to be a really neat idea. Maybe it could even be a (semi) official Obsidian project?