don't forget about ebay and/or half.com... i found a bunch of my books through there... oh yeah, and dont discount older editions of books... I found a few of mine pretty cheap because they were the last edition... and most of what changed was addition of color to a few of the pictures!!! We found out I think in May what books were recommended/required which gave my plenty of time to scour the net for good prices
Also, I am not sure how it is at other schools, but our vet bookstore (different from the whole campus bookstore) has pretty good prices, often cheaper than amazon (especially with our 10% SCAVMA discount... though you have to pay in cash/check to get that full discount)....
And one more thing... don't get too carried away with buying books unless you have lots of money... they add up really fast and for a few classes I NEVER opened the books... and if you find out that you do want the book after class starts, you can always buy it then!!