School time again, huh?
As a student I always hated writing in books - especially since it would lead to books being sold back to the bookstore for less money.
Post-it came up with a product that you can use that allows you to write in books, yet at the end of the semester still sell the books back as new. They are called "Sheer Colors Post-It Notes" - they are basically see through Post-It notes that you can use to write on the post-it and reference the stuff underneath.
I personally still use regular post-it notes at the moment, but I totally would move towards this when I get rid of the supply of post-its that I have.
Link to an article about this