Your old book should have been migrated if it was on the drive at the time that you installed ADE 1.5. Odd that it wasn't...
Anyway, if you want to go back to the old AdobeID, launch ADE, then press control-shift-D. This brings up a dialog that allows you to deactivate. Do that..
Then relaunch ADE 1.5, and choose 'Authorize Computer' in the library menu and authorize with your old id..
If you get stuck open a web support case at http://www.adobe.com/support/contact.
Regards,
Bentley Wolfe
Senior Support Engineer, Flash/Flash Player/Digital Editions
Adobe