Why are my iBooks no longer available or removed from my iBooks App?

Whenever Apple does an update to the iPad, something seems to go wrong with downloaded apps or products.  Sometimes your iBook will need to be downloaded again from your purchase history.  You DO NOT need to buy it again. Simply, go to the App Store or iBook Store and click on "Purchased" at the bottom of the screen. Select the items that you would like to download.
Occasionally, Apple will lose its distribution license for a particular item or app. If you have not already redeemed your code, your code will become invalid.  If you have already redeemed and downloaded your product, Apple reserves the right to remove it, but will likely allow you to continue using it.

Please contact their support team at 1-800-275-2273.

Due to Apple restrictions, you will have to contact Apple directly for support.