https://www.gamespot.com/articles/save-money-on-books-during-amazon-prime-day-2018/1100-6460426/

Everyone knows reading is good for you. If you're the type to crack open the occasional book, you'll want to take a glance at this deal at Amazon that's good through the end of Prime Day. This special offer gives you $5 off any book order of $20 or more. That's up to 25% off, and it's in addition to any discounts Amazon already has on the books in question. To get the savings, you have to enter promo code PRIMEBOOKS18 at checkout. The deal expires at 11:59 PM PT July 17 / 2:59 AM ET July 18.

It wouldn't be a special offer without some good old restrictions. This deal only applies to physical books (sorry, Kindle readers), and it must say "sold by Amazon.com" on the books' product pages. Also, the promo code is only good one time per account, so use it wisely. You can read the full terms and conditions here.