The issue of purchasing textbooks is like many things in life an issue of timing and risk analysis. If you wait until the class starts (your safest approach because you might drop the class) you risk on missing out on all of the used copies of the required book, both in the university bookstore, and online. If you buy the book too early, you may lose out because you may drop the class. As always the issue is one of timing. If you plan to purchase a used copy of the book at your school bookstore (something I do not advise) then you have to wait until the first week of the new semester, since most university bookstores do not put books out before that time.

If you are planning on looking for a deal for your book online, you have to expect at least a week and a half for the book to be shipped, so if you are expecting an assignment on the first day of class, then you will need to either share with a friend, or use the library reserve copy. You can always choose to pay for express shipping, but that tends to nullify much of the savings you'd otherwise have. Also, note that price comparison sites tend to display the price of the book with the cheapest shipping.

As far as the best time to buy the book in order to get the best deal I would recommend buying the book 1-3 weeks before the start of the class. This should ensure both that you get the lowest price (because you will get the cheapest books before they are sold out) as well as giving you enough time to receive the book in the mail. You may still drop the class but in that case you could always sell the book the next year or semester.

Another important thing to note about looking for the best price for a book on price comparison sites is that many of the price comparison sites archive their results in order to speed the search. This greatly diminishes the quality of the returns, and makes it very difficult for you to find an up to the minute deal. At We Compare Books every search you do is live, and although this may lead the search to be a bit slower this is the only way to ensure that you are getting the most accurate price information.