I, too, recommend Amazon.com ... Now, if you prefer getting a "new" book, go to Booksamillion.com. They have a "preferred shopper" card for $5 and it gets you 10% off ALL purchases, including college textbooks.
My wife and I are both attending grad school and have really enjoyed our BAM experience

They get books out quicker than B & N and Amazon.com.
If you do purchase your books at booksamillion.com be sure to clickthrough ebates or another shopping portal to get even more back
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