The principle that all libraries everywhere are connected by the bookwormholes created by strong space-time distortions found in any large collection of books. Since Knowledge equals Power equals Energy equals Matter equals Mass, a good bookshop is just a genteel Black Hole that knows how to read. The three rules of the Librarians of Time and Space are: Silence; Books must be returned by the last date shown; and the nature of causality must not be interfered with.