I have a tendency to underline when reading books. Well, I should say when reading philosophy, theology or other non-fiction. Although, I do occasionally underline in fiction as well if I find a quote I particularly like.
This is my method (and a very common method) of finding the things that stuck out to me without having to search or reread the book when I'm looking back at it.
Have you ever read someone else's book? One that's already been highlighted and underlined...creased and worn. In college this was occasionally handy if the person who had the textbook before you happened to be a very good at picking out the important information. More often than not it was more distracting and annoying, especially when you get those "excessive highlighters" who insist of highlighting entire pages.
But when you borrow a book of substance from someone you know personally, it can be a really interesting experience.
I am currently having this experience with a book that I borrowed from a family member. You learn a lot about someone when you see what parts of a book stuck out most to them and what they identified with or were convicted by in the pages. You may find out what you have in common, or what makes you different.
I'm somewhat of a book collector (meaning I usually purchase) so I've only had this experience one other time, and that was with my husband...back when we were dating. We happened to accidentally start reading the same book simultaneously during a time when we weren't in contact. We later went through and read each others books/notes and got a picture of what the other was going through and learning during that time. It was a wonderful gift.
I guess all of that to say...
I hope you get the chance to experience reading someone else's book.