I have highlighted and marked books and I tended to buy the cheapest copies possible (or buy used books) as I was quite tight on money when I was at university.

What I discovered was that I could trace my understanding and my development towards concepts through the times when I returned to a book that I already had worked with before. My older notes and highlights were often the starting point but with my then current understanding I built on them, contradicted them in parts and sometimes laughed about my former ideas. It made me more humble towards knowledge generation.

To me it were a blessing to have different layers of marks, notes & highlights.