I really really hate it when I take out a library book only to find that some doorknob has written in it — I don’t care if it is just neat little underlines or asterisks in the margin or dumb comments. I also don’t care if you intend to erase your pencil marks when you are done — it still damages the book. If you want to fold, spindle, or mutilate a book, then buy it yourself!
If you write in library books considered yourself on notice! And stop it!
(And for what it is worth, I personally only make very minor marks in pencil in my own books)