Kindle books are already cheaper than physical ones (Print is dead. Blah, blah, blah), but with this Prime Day deal, you can get even more money back into your wallet. Buy any Kindle eBook and get 40% back to use towards your next Kindle purchase (up to $20). You’ll need to use the credit by 7/25, but I’m sure you’ve had more than one book you’ve been wanting to read this summer.