"Do you write in your books? Highlight? Make notes? Or do you like to keep your copies as pristine as possible?"First, let me say that I teach from books. It helps to have notes about or at least highlight sections of a book I may want to quote. Friends who have Kindles assure me I can make notes in them, but I have noticed they cannot quickly find what they need. And they can't flip back and forth easily. So yes, I do write in the margins of my books, including my Bible. I underline, occasionally highlight, definitely make notes in the margins, and use post-it notes to guide me to important points I want to share while teaching my students.
On the other hand, I rarely mark in novels mostly because (1) I rarely teach from a novel and (2) I rarely keep my novels. In fact, if my library has a copy, I usually borrow novels I want to read. Naturally, I can't mark in library copies, though I may annotate them using temporary post-it notes. The only people I know who keep "pristine" copies of books are collectors. Although I "collect" books in the sense that I actually have thousands of books, I don't collect books just to have them. I use books for a specific purpose:
for the ideas I find in them
for sharing the knowledge gained