Some books I definitely do highlight and underline passages, but most I don’t. Books for entertainment I definitely don’t highlight or underline. My copy of A Party of One: A Loner’s Manifesto (which I emphatically recommend) is filled with notations, highlights, underlines, bent corners, you name it. I even wrote a list of my favorite pages on the inside of the front cover. I’ve read this book countless times over the past five years or so and I still identify with everything I’ve marked up. Maybe if I gave it a break for ten years or so and came back to it I wouldn’t. I don’t think I could stay away that long though.