Amazon is running a coupon code today where you can take $5 off of $15 in books. Here's some of the fine print:

Enter promo code PRIMEBOOKS17 at checkout to qualify.
Offer only applies to products sold by Amazon.com (US only) - look for "sold by Amazon.com" on the product detail page.
Offer does not apply to digital content.

See all of the details and get your coupon!