My college had a Reserves section in the library where professors could request that several copies of popular or rare books required for their classes would be kept, and could only be checked out for 2-3 hours at a time. It was a good option if you didn't want to buy every book on the reading list. Sometimes there was a wait, but you knew you would be able to get access to the books.

Some professors are also moving towards e-Reserves systems/paperless . . . depends on the subject area and whether it is okay with publishers, but in my experience that can save a lot of money, too.

It's also possible to set up informal book sharing arrangements with other students, or bypass the campus bookstore by buying used directly from another student, i.e., cut out the middleman. Just have to start talking to people, maybe post a want ad in the right place?