Well, the answer to this depends on your memory. Do you remember if you used named activation when you originally purchased the book?
If you used 'Easy Activation' (otherwise known as 'anonymous activation') then you're in trouble, because books activated that way are NOT portable and cannot be moved to another drive or machine.
Now, if you'll open a support case then we can check to see if there are activations against your email address. Also consider the possibility that you may not be remembering the address you authorized with correctly..
(To open a Digital Editions 1.5 Technical Support case, please use the contact links at http://www.adobe.com/support/digitaleditions)
Regards,
Bentley Wolfe
Senior Support Engineer, Flash/Flash Player/Digital Editions
Adobe