[PeterT]

I think that first of all you have to use codes via the website, not from the device itself.

Secondly, you can't use codes on all books; the standard means of telling if a book is eligible for a promo code is to look at the price on the website. If the book says something like

$11.99
List price: $18.95
You save: 37% ($6.96)

then, when you get to the payment page on the website you will see

REDEEM ANY GIFT CARDS OR PROMO CODES
Have a gift card or promo code? Use it here

When you expand that selection you can enter the promo code.