The first rule of Hollywood is "Nobody knows anything".  That is basically William Goldman's mantra in his groundbreaking memoir.  The book got Goldman in hot water with some people as he pulled no punches, gave examples of stars behaving badly, and also was merciless on some films, almost giddily pointing out their flaws. 

But perhaps the most interesting part of the book is how Goldman almost seemed to predict the current Marketing Era of film.  While Goldman didn't foresee the script process being as rushed as it is in many cases today, he correctly predicted that sequels would become all the rage and that pre-sold audiences would become more and more important to studio heads (as opposed to quality which would draw in crowds). 

Be sure to pick up the copy of the book that comes with Goldmen's script of "Butch Cassidy & the Sundance Kid".  The book is a quick read but will definitely stay with you.