I am always interested in expanding my theatrical vocabulary and knowledge and am curious about what others consider the essential, must read books about theatre.  Whether they be about acting, directing, stage craft, stage management, theatre history, etc, what books do you consider as fundamental reading about our craft?  
Thomas Kelly's Backstage Guide to Stage Management  and Lawrence Stern's Stage Management  top my list for SM books, and Uta Hagen's Respect for Acting  is one of my favorite acting books.