Not sure if this is only applicable to students, but I just got this email:
Dear Amazon Student member,
We're offering Amazon Student members an additional $10 off already discounted Kindle e-textbooks now through January 9. Kindle e-textbooks can be read on your iPad, PC, or Mac — no Kindle device required. Some restrictions apply. See terms and conditions.
I used the link and added the $10 credit to my Amazon account with no troubles.
Eligible e-textbooks will have a little box saying so.
Here's one you can get for free which is a bestseller:
Jean-Paul Sartre: Basic Writings http://www.amazon.com/Jean-Paul-Sartre-Basic-Writings-ebook/…
Works for all Amazon accounts. Just click the link, sign in, and enter the code following the instructions.
Worked for me and don't have a student account.