While the rules for fair use are not black and white, one of the primary tests is whether the copying impairs the market for the work. If you want to copy pages of a book to mark them up, for instance, so your original copy stays clean, that would generally fall under fair use. You aren’t selling the copy or the original. You aren’t giving one or the other to other people, thereby eliminating a potential sale. You are copying some pages, not the entire work, cover to cover. As you say, you wouldn’t get in trouble for it in any case, but I’m pretty sure that it would be covered under fair use. But yea, if you photocopy a book and give it to your friend, that’s illegal.
