MacInsiders has a few articles on buying books, and some advice about buying them.
- Buying Your Textbooks (new and used)
- Used Books = Save Money!
- Where to get my books?
I would recommend waiting until you get your official booklist (as they change each year) and then see if your friend has the textbook(s) you're needing.
If you buy all the books off of your friend now, and then come September find out that the book they sold you is no longer being used, or a newer edition is being used, then you just wasted your money. However, when you get your book list and see that they are using the same textbooks as last year, then buy them off your friend