As for buying old books, I know that Blackwells sell 2nd hand law books at a cheaper price but really, the more up to date a book is, the better it will be for you as even the most up to date books are out of date before long due to the ever changing nature of law. Some books have companion websites though i.e. you can go online and there will be updated information that you can download, that isn't featured in the book.