"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."