MOST Textbooks (especially my accounting texts)...highlighter like a road map...different colors for different things I want to emphasize. Notes in the margins, notes on any blank area, inside the front/back covers, etc. THOSE are books I knew I'd be keeping for reference later.
The ones I knew I would be selling however, not a mark anywhere.
Private reading material such as novels, not usually. If there's something I find of particular interest, I MIGHT use a post-it note stuck inside the front cover.