I just registered yesterday for my final semester in Humboldt State's Teaching Writing program (graduation is May 15!). And it reminded me to start looking for books. For a few years, I've been using this useful website, www.paperbackswap.com. The concept is simple: you send people the books you no longer want, you receive new books that you do want.
Basically you post a list of what you have. If someone else sees something they like, they request it. You pay for postage to ship to them (generally around $2.50 or so via media mail) and then you get a credit. You can use the credit to request any of the millions of books other people have listed. And they ship it to you for free (well, free to you).
One of the best features is that you can create a Wish List for books that aren't posted as of yet. Then at the moment someone does list the book, they send you an email asking if you want it (provided you're at the top of the waitlist). You have 48 hours to claim it, or they ask the next person on the list.
I've found it a great place to get the books for my classes. Of course, it works best for literature classes, as every fifth person has a Charles Dickens or George Eliot novel they want to part with. But I've been able to get a few textbooks for other classes out of it too. So right about now is when I should email the professor to ask for a reading list (because if I wait until I get the syllabus on the first day, it may be too late to get a book in time for the first assigned reading).
Check it out at www.paperbackswap.com.