Authorization issue

Call me old but I cannot figure out this technology thing.

Problem. I bought a textbook that can only be read on Adobe Digital Editions. I would like to have access to that book on both of my computers (iMac and Macbook). I have the book loaded on ADE on my Macbook and it works decently. How can I get, or can I get that book on my iMac as well?

Thanks,

Alex