For iBooks, it is not uncommon that the code did redeem on your account, but failed to complete the download process. Apple often returns the message that your code is already redeemed. Go to your "Purchased" items at the bottom menu bar of your App store. Then find the product that you are attempting to download. Click on the product icon to complete the download process.
If you have verified that the access code has not been redeemed on your account, please submit a support ticket by using the help tab on the right of the screen. We will get you taken care of, although the agent will likely take you through the process mentioned above before issuing a new code.