I don't annotate books that aren't mine. However, I was wondering about it. I am fairly competent at underlining (always in pencil) books and making annotations that aid in recall. Suppose you are in a college library and you know a section of books isn't used (which is more and more the case). Knowing that a book will probably never be checked out, and knowing that even if it is checked out, your annotating in it will actually help the future reader, are either of the following scenarios wrong: a) Underlining in pencil. b) Underlining in pencil but erasing before turning it back in.