… Which is not meant in any way to imply that librarians aren’t needed, or don’t have far more to bring to this discussion that your humble &c. &c.

Thanks for you interest, too. I think that bibliographic description and subject classification are things that libraries do very well and are fundamentally necessary, but we absolutely make a mess of presenting them in a useful way to the people that they’re supposed to serve.

In a conversational sense, you’re entirely correct. When we say “book” we usually let the artifact stand in for its content and vice versa. In creating a library of any sort, though, one has to identify distinct “bibliographic entities” (this copy of that edition of this work by that author). The Anglo-American Cataloging Rules (AACR2) are one system for doing this, the Functional Requirements for Bibliographic Records (FRBR) are a conceptual model for the whole process.

Those individual books might be arranged in physical space by one or more methods. Size, subject systems and date of acquisition are the most common (and I’d assume it’s the first and last that predominate in digital libraries) but they’re referenced through any number of subject classifications. Moreover, as any given book can have numerous subject headings, none of them serve to uniquely identify it.

There’s another level of distinction between a subject heading and a call number (or shelf mark). The latter often do incorporate a subject based number, but have to add additional information to actually address a particular book: eg. 917.3 might be the DDC number for 372 books in a given library. Adding a Cutter number for the author’s name or the title of a work, and a copy number if there are multiple copies, is necessary to actually address a distinct book: 917.3 C34 D53 c. 2. (Even then, most library systems actually track books on a unique key that’s invisible to the user.)

That still won’t tell you what that particular book is. That is, there’s no reverse decoding of a call number possible to get a bibliographic description. The same is true, of course, of ISBNs. One can use them as a key to look up individual works, but that key still has to reference a bibliographic description.

Machine readable cataloging (MARC) is a format for storing all of that data, used by libraries for the last 30 years or so.

LoC and DDC are subject classification systems, not systems of bibliographic description. The library acronyms that I think you want are MARC, and to a certain extent, AACR2 and perhaps FRBR. But I’d argue that these latter two, FRBR especially, will actually have to be among the core documents in the process of “consolidating and clarifying” that you mention.