Well, the best way to answer this is to quote (with permission) from the Discworld Companion:
Libraries, nature of.
Even big collections of ordinary books distort space and time, as can readily be proved by anyone who has been around a really old-fashioned second-hand bookshop, one of those that has more staircases than storeys and those rows of shelves that end in little doors that are surely too small for a full sized human to enter.
The relevant equation is Knowledge = Power = Energy = Matter = Mass; a good bookshop is just a genteel Black Hole that knows how to read. Mass distorts space into polyfractal L-space, in which Everywhere is also Everywhere Else.
All libraries are connected in L-space by the bookwormholes created by the strong space-time distortions found in any large collection of books. Only a very few librarians learn the secret, and there are inflexible rules about making use of the fact - because it amounts to time travel.
The three rules of the Librarians of Time and Space are: (1) Silence; (2) Books must be returned no later than the last date shown, and (3) the nature of causality must not be interfered with.
It can easily be seen (I hope) how this idea maps onto the WWW, with its links from here to there to elsewhere.
The L-Space Web is a creation
of The L-Space Librarians
This mirror site is maintained by A.H.Davis