Marking up books.

I used to think people shouldn't mark up library books, and I still think they shouldn't, if they don't know shit, but I really appreciate analytical library notes in books I'm reading by people who know what the fuck they're talking about. So, all this week I've been reading this reserve book from the library here on campus and taking extensive notes on it, since it's a pretty famous recent book that's a total piece of shit, only you wouldn't notice it unless you were in the field or someone pointed it out to you. I guess I shouldn't mark up books because the library says I shouldn't, but I feel that I should be able to violate the rule on principle, and since I have those principles, I do. Does that make me a dick?