Having txt files is just more portable for me in terms of any future software changes. It’s a backup for lifetime essentially as I’m not tied to Apple Notes nor Obsidian.

I save more by default too, I have a template that imports when I add new book to my collection and I just fill it with info/tags and my notes/quotes and other interesting stuff related to the book.

My favourite thing about Obsidian is that you can interlink all the notes together. Its super useful if you’re referring to other books in your notes or if you have literature notes on different books.