The gift certificate offer is here:
UK,
US
Buying $20 of gift certificates gets you $2 in reward dollars. Combine with the 50% off RRP, and you can buy $44 RRP of books for $20, a 55% discount. And then you get back another $2.20 in rewards. So that's $44 of books for $17.8, a total theoretical discount of 59.5% off RRP. That does seem an excellent discount.