A BIZARRE PHENOMENON first observed in the 1940s became a crime-fiction epidemic by the 1990s. Famous entertainers, athletes, and presidential relatives began sitting down at the typewriter to bang out mystery novels. Or so they would have us believe. In truth, nearly every one of those celebrities made a deal through an agent or book packager, collected a nice advance for the use of the name, and left to a professional ghostwriter all the actual writing. Ghostwriting is a time-honored practice, and most readers surely realize that movie stars and baseball players have help with their memoirs--just as all politicians...