Are you redeeming a gifted book, or redeeming a gift card so you can buy a book?
In the second case it makes no difference if you redeem the code in iTunes or the iBooks store. It all goes to the same account.
If a book, you should have received a link via email. Tap on this in the Mail app and you should be, semi-automatically, taken through the steps to download the book. It also might work if you tap on the link in Safari, if viewing your email on a website. I've never tested that one.
Edit: Or better yet, read the help page Johanna linked too.