ELAG 2010 featured a “Workshop on FRBR and Identifiers”. The presentation gives an overview of which identifiers exist for various forms of resources, with special emphasis on FRBR entities, and including a brief look at the role of identifiers in linked data. Just for completeness’ sake, I won’t talk about URL identifiers for FRBR entities and relationships here – a vast topic in and of itself.
Library-created control numbers identify the metadata about the resource, not the resource itself (like ISBNs). For one resource different institutions (publishers, booksellers, libraries) create different identifiers – but how reliable and consistent are they? One ISBN doesn’t necessarily stand for one book only, undermining uniqueness in many cases. As WorldCat data shows (assuming that catalogers correctly recorded the details available), we have a large number of books without ISBNs (which only came into widespread use in the 1970s). Generally there is a considerable percentage of resources which are not identified in a standard way. So the picture is not uniform at all, and some of the established identifiers will have to be reconsidered: the ISBN system is likely going to reach its limits with the proliferation of e-books, and maybe the library world will sometime stop thinking in terms of “records” (possibly with metadata being assembled just in time instead of just in case) – will the LCCN be obsolete then?
There are many efforts of creating and maintaining identifiers in different domains. Libraries around the world maintain separate authority files (albeit tied together in VIAF) and create separate “records” and thus identifiers for the same resource. It’s important for identifiers to be reused outside their specific areas. Library identifiers have lingered in silos for a long time and are only slowly being adopted by “outside” communities (e.g. German Wikipedia linked identifiers from the National Library’s name authority file with the articles about the respective persons).
A given FRBR work usually has various manifestations which in turn have several identifiers (leaving out the expression level for the moment) – those are the most commonly used (ISBN, LCCN…). OpenLibrary, for one, collocates manifestation identifiers. Topic maps could integrate information from heterogeneous sources on the basis of identifiers. We can probably never achieve global agreement on one unique bibliographic identifier, nor do we have to if we have systems that enable us to consolidate the diversity of identifiers.