One thing I learned in Metaphysics class: a book is not properly read unless its margins are thoroughly written in. The prof used to walk up and down the aisles checking our textbooks to see if we had been valiantly underlining and scribbling. He was the youngest in the room, BTW.
Can't do this with library books. So Amazon is my library now. Besides, they have a bigger selection than even the university library. Now that I am retired and have eliminated most expenditures, I find that I can afford way more books than I can read no matter how I let my specialized selection evolve.
So that if you discover an elegant mathematical proof of something, you should make note of it in a textbook, but never get around to actually writing it out?
No thanks. I don't write in my books. The Prof can flunk me.
I also don't dog-ear the pages. I've discovered they all have numbers on them.