I also emailed amazon. They responded with an email saying they have given me the credit to purchase the book. I didnt get a code. I followed the link. I proceeded to order the e-book. Nowhere did I see a $3 credit. The transaction went through. I checked my order details and it had "promotion applied" under payment for $2.99. It makes me think that the promotion was tied to my account because of my preorder and amazon just provided the link and told me kindly to purchase the book.
Either way. If you preordered the game and you haven't gotten an email about the e-book, decided to purchase it and was charged for it, just email amazon. They are very generous with refunds.