I ran into the same problem while building https://learnawesome.org/ . Forget the broad class of "learning resources", even the "books" category doesn't have a usable unique ID. Not everything gets an ISBN for example. There's also the ambiguity between a "work" and an "edition" of a work.
ISBN is an interesting case as the publisher has leeway on how it's handled accross the lifetime of a book. In particular some books will get multiple ISBN for the same version, and some other books can keep the same ISBN through multiple minor revisions.