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Electronic versions of textbooks are often cheaper than their printed equivalents, and are available from most major retailers. If you don't find the title you are searching for, check the publisher's website.
Be sure that you're getting the right book by checking the title, author and edition/year. If you have it, use the ISBN to locate the correct edition for a specific class. The ISBN is a 10 or 13-digit number that can usually be found on the barcode of a book. This number is unique to that title and is the quickest way of finding the book you need.