This offer is only for those with a Kindle with Special Offers:
Save $25 when you spend $100 on textbooks at Amazon.com
Click on offer, then click on the link on the offer page to receive an email with the promotion code. Sign-up for this offer expires on August 16.
You'll get an email (right away), a link to the Special Offer page and a promotion code to enter at checkout. Once you have the promotional code, you have until September 16 to complete your purchase. Like previous offers, this one requires you to use the full checkout process in order to enter your promotional code. Also, like all Amazon sales that use promotional codes, if you have a gift card balance, you must use it for the payment (if there is not a sufficient balance, then you can pick which credit card or other payment to use).
This offer can be used more than once per account, so long as you have more than one KSO registered. Just click on the ad on each device to receive the individual promotion codes via email.
This offer only applies to new textbooks sold by Amazon (not any third-party sellers) and excludes Kindle textbooks. But, for those who need a textbook or two (or some books that Amazon has classified as one, even if they aren't your typical "textbook" types of books), it's a decent discount. I just glanced thru some of the categories and there are a number of non-fiction titles, such as various computer related texts, business books, science and law titles, etc., that are of general interest.
Update: Amazon just added a Get $5 in MP3s with the Purchase of $50 or More in Textbooks deal that you can combine with this offer. Best of all, you don't have to do anything at all - if you buy $50 or more in textbooks between August 15, 2011 through 11:59 PM PST on September 16, 2011, you'll get the MP3 credit automatically, with confirmation via email.