Ranganathan's five 'laws' have classic status in the library community. I am using 'classic' in the Frank Kermode sense of being 'patient of interpretation' over time: they express something which remains relevant as contexts change.

I wrote about The Long Tail in terms of aggregation of supply and aggregation of demand.

In this context, aggregation of supply is about improving discovery and reducing transaction costs. It is about making it much easier to allow a reader to find it and get it, whatever it is. Or, in other words, every reader his or her book.

Aggregation of demand is about mobilizing a community of users so that the chances of rendezvous between a resource and an interested user are increased. Or, in other words, every book its reader.

Incidentally, for those who need to be reminded about the full five laws, check out the stylish Quædam cuiusdam.