Do you still have a version of ADE running that can read the books?
If so, click ctrl-I on a book and it will give various info including registration info.
Old ADE will only give the email address, new one will give the email and also the internal ID.
If that doesn't work, you can peek inside a book and see.
It's quite complicated: I'll give the instructions if you find your really need them.
Unfortunately, that will only give you the internal version of the Adobe ID, not the email name currently associated with it.
I'm not sure if Adobe help will be able to tell you the associated email address, or even to reset the ID with a new one, if you only have that internal ID.