How to redeem an iBooks promo code on your iPad or iPhone?

Step 1: Find The Redeem Button
1a.For that, you should open the iBooks app (on your iPad or iPhone or iPod Touch). iIf don’t have the app installed on your device then click here to install the iBooks app – its a free app.
1b (optional). If you’re on the “Library” mode, browsing the ebooks you already bought, touch the “Store” button and make sure you’re on the Apple store and not on your local device. 
1c. Once you get to the iBooks Store page, touch the first tab button in the navigation tab bar on the bottom of the screen. It is the one called “Featured”. Now scroll all the way down to the bottom, and voila: the “Redeem” button magically appears.

Step 2: Enter Your Promo Code
Now, after touching the “Redeem” button, you’ll be taken to a new screen, pretty much similar with the one below:

After you entered your promo code, you will (hopefully) get a message saying that your item is downloading. What iBooks does is that it automatically matches the product with your promo code, looks it up in the store and starts downloading it. In a few seconds, if you have a good internet connection, your book will be available in the Library.