Unfortunately our discount feature cannot work in the way you describe, because we have no idea who your buyer is until they've made their purchase. At that point it's too late to judge how many times they have used a given discount.
If you want the discount to only be used when the buyer has at least 8 books, set the minimum quantity to 8. From there you could also add an expiration date so that the discount can be used over and over during that time as long as the buyer has purchased at least 8 books.