You can't transfer DRM eBooks between accounts.
Even if the old email is no longer in use, the AdobeID associated with it should still work.
You may have problems if you have forgotten your password though; may need to try Adobe Live Chat: http://www.adobe.com/support/chat/ivrchat.html
Once you manage to login to an old Adobe ID, you can change the email address associated with it.
The ID remains the same, just the email address you use to access it changes.