The truth is that even big collections of ordinary books distort space, as can readily be proved by anyone who has been around a really old-fashioned second-hand bookshop, one of those that look as though they were designed by M. Escher on a bad day and has more staircases than storeys and those rows of shelves which 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.
- Guards! Guards! by Terry Pratchett, pg. 11