Like I said before, you honestly don't need to buy any books. Half the information you can find online or in your lecture notes, and most of the books' older editions are available as eBooks (just use google, you'll find them).
And if you really really need to use a book, then go look in the library, you will almost never find that all copies of the book have been taken out. Our library at Brighton also has some desk loan copies which are kept in the library at all times and are available for a couple hours at a time, so if your school's library has that, then take advantage of it.
Also, saves you the trouble of having to lug a heavy book around with you