Just as books could not easily be distributed en masse (no trucks or planes or railroads), they could not be produced en masse (no printing presses). And since they had to be copied by hand, one at a time, slowly, painstakingly, most books were not mass produced. Those few that were produced in multiple copies were not all alike, for the scribes who copied texts inevitably made alterations in those texts—changing the words they copied either by accident (via a slip of the pen or other carelessness) or by design (when the scribe intentionally altered the words he copied).
