A few ideas: if you borrow from a library, use sticky notes for notations. I used to do this with books throughout college and it's a pretty effective way of writing down thoughts about the book, but then you can keep them and return the book when you're done with it.

If you like it enough to want to buy the book, check out local thrift stores and buy them second-hand. It's gonna save a bunch of money. You can then put the sticky notes into the second-hand books.