Yes I mark mine up, surprised so many here don’t. I used to be a person that never did, but heard some people on podcasts highly recommend it, and I also began wanting to take notes. I think it adds value to the book on a re-read if you do it cleanly. I underline the first and last word of the highlight, with a curly bracket in the margin to indicate the area (sometimes a comment added), and a small plus sign in the top right corner to indicate which pages are noted. Then I can flip through when finished and dictate the notes to my computer. But they also make sticky tabs for page notes if you don’t wanna mark books up. I do have some visual or big coffee table books, like Poor Charlie’s Almanack, that I don’t want to mark up inside.