Ugh. This deal isn't as great as it is made out to be. If you buy an $8 book, you have a $1.20 "reward"
If you go buy another $8 book, your net is $6.80. You get 15% off of that, so your next reward is $1.02. That isn't a 15% reward off of the $8. it is only 12.75%.
And to make matters worse, if you have to pay sales tax, you pay it both times on the $8.