The library didn’t only contain magical books, the ones which are chained to the shelves and are very dangerous. It also contained perfectly ordinary books, printed on commonplace paper in mundane ink. It would be a mistake to think that they weren’t also dangerous just because reading them didn’t make fireworks go off in the sky. Reading them sometimes did the more dangerous trick of making fireworks go off in the privacy of the reader’s brain.
–Terry Pratchett. Soul Music