cannot authorize more than one device

Hi, I have bought an expensive book and I would like to have access in more than one of my devices. First I was able to install it in my macbook without authorization. But now I want to continue having access to the book through the iPad and cannot manage to do it. I figured it was because in the iPad I had authorization and in the macbook i didn't. I tried to solve that just to find that message, that I cannot authorize more than one device... the point is that now I don't know how to access the book from the iPad.

Any ideas?

Thanks