Discussing grades of availability in my last post, I mention an article I wrote a few years ago on libraries and the long tail. Here is how it starts:
Discussions of the long tail that I have seen or heard in the library community strike me as somewhat partial. Much of that discussion is about how libraries contain deep and rich collections, and about how their system-wide aggregation represents a very long tail of scholarly and cultural materials (a system may be at the level of a consortium, or a state, or a country). However, I am not sure that we have absorbed the real relevance of the long tail argument, which is about how well supply and demand are matched in a network environment. It is not enough for materials to be present within the system: they have to be readily accessible ('every reader his or her book', in Ranganathan's terms), potentially interested readers have to be aware of them ('every book its reader'), and the system for matching supply and demand has to be efficient ('save the time of the user'). [Libraries and the long tail: some thoughts about libraries in a network age]
Incidentally, I think Ranganathan's 5 laws [Wikipedia entry] remain relevent in lots of ways to current discussions, as above.