I'd only buy a few basic books. The absolute necessities. Everything else you can get from the library (they'll probably have copies in the short loan section that can't be booked out for longer than an hour, so you'll be able to get hold of them).
As I got further into my course, I found I used journal articles for the vast majority of my reading. It might be different in courses that aren't science related, but journals are the bread and butter. Out of the 70 or so references in my dissertation, I don't think a single one was taken from a textbook.