I recently overcame this as I've been reading more of the unread books populating my shelves. I always wanted to keep my books in pristine condition and never wanted to mark them up. Now, I realize they are part of MY collection and marking them up with highlights, underlines, notes, etc., only make them more valued since they contain their intrinsic knowledge from the author as well as my own views. Even if I want to give them to friends to read, they'll see areas that jumped out to me and maybe appreciate some of the additional context or focus I noted as well.