what is the difference between an ebook and any other type of electronic document that might be published or otherwise circulated?

Nothing!

Therefore, in terms of identity the real difference is between documents meant to be circulated publicly (published) and those that circulate privately (between individuals).

Published material then differs between what is intended to be sold and those freely circulated, and then those copyrighted and in the public domain.

In other words unique identities are needed immediately and on-the-fly, then followed up by appropriate registration on public data bases.

The idea that registration is connected with issuing ids is a non-goer as the report above shows in the handling of this non-issue.