Some years ago, I was told that the adhesive on post-it notes can stick to pages and damage books (and other documents you want to keep). I don't think they've changed the adhesive, but I don't know whether this is the sort of thing that's bad to leave on for months and okay for a couple of hours.
I use post-its mostly to leave notes attached to my computer monitor, or scribble things on them, fold them so they won't stick where not wanted, and tuck them in my pocket or wallet. I don't know whether I'm being unnecessarily cautious with books, or taking good care, especially with library and other borroed books; cheshyre will either have seen more information or know where to look.
FYI: Post-its
I use post-its mostly to leave notes attached to my computer monitor, or scribble things on them, fold them so they won't stick where not wanted, and tuck them in my pocket or wallet. I don't know whether I'm being unnecessarily cautious with books, or taking good care, especially with library and other borroed books;