I think it's worth exploring with some care what URIs were meant to be, and how they are being used in practice. In particular, I want to look at what illumination might be contributed from our understanding of how other kinds of identifiers work.
When I think about the library "catalog" I think that it will contain more than just metadata about library materials but also the materials themeselves. Think mostly books, journal articles, encyclopedia articles, definitions, images, data sets, etc. Moreover, the library "catalog" will enable people to do things with the items in the collection. It is more than just find and get.