Brandon Sanderson is the author of the wildly popular Mistborn series (and a series that I liked even more - Wax and Wayne). He's also released a couple of his unpublished books. One of them he had recorded as an audiobook. Dragonsteel Prime, which I listened to. He wrote it as his honors thesis. It was fascinating to listen to. He said that in hindsight, he was glad it wasn't officially published as his debut novel, because he still had a lot of growing left to do as a writer. I could see what he meant.
I don't think I've ever read an unpublished novel by an author that I'm familiar with. It's amazing to see how their ideas evolved. His writing skill was amazing back then. His plot/characters/worldbuilding were good, but they did leave room for improvement.
I wonder if I'd be brave enough to ever release some of my older stories that I've since abandoned...
Vinny, my la mancha goat buddy.