Q: how do i un-redeem an itunes card?

iTunes gift cards are redeemed onto an account, it's balance will be available wherever you are using that account e.g. iTunes, app store, Mac App Store and iBookstore - so if you are using the same account in the iBookstore then you don't need to do anything.

If you are using a different account in the iBookstore then if you've only just redeemed the card and not spent any of it then you can try contacting iTunes Support and see if they will transfer or unredeem the card : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption