I've come across them myself a couple times and they're always a pleasant find!

I generally read library books on my kindle, and there's the highlighting function that automatically words and passages. I can access those even if I don't have the library book checked out.

If I read a physical library book, sometime I use sticky notes and leave my notes in there for the next person. I'm not ruining the book, and it's an interesting little find for the next reader :)