I not only didn't hold onto my books, by the time I was graduating I would figure out how many pages of the book the professor actually wanted us to read and made photocopies if it was more cost-effective. I got so annoyed at professors having me buy entire books to read one chapter. I could spend $5 or $10 in photocopies and be miles ahead of buying even the cheapest of used books. It took some quality time in front of the copy machine in the library, but it was worth it!
I'd be scanning the various e-book sources as well for any alternatives to those ridiculously overpriced books. But that wasn't an option for me in the 80's.
And ADPiUCF is exactly right. Unless it's a Shakespeare text or the Human Anatomy book that every pre-med major has to have, I wouldn't keep any reference books. They all change too much to worry about.