How to move ibook from my iphone to her ipad?

I bought an ibook for my wife to use on her iPad. It downloaded onto my computer and is now on my iPhone, but I want to instead put it on my wife's iPad. If I just copy the file over to her iTunes library, the sync fails with an error that the ibook is not authorized on this device. How can I fix this? I only want the ibook on the iPad - nowhere else. Thank you.