Want to know the difference between "O.S." (off-screen) and "V.O." (voiceover), and when to use each? How about that between "Intercut" and "Match Cut?" Then you want to buy "The Screenwriters Bible," script consultant David Trottier's encyclopedic reference on creating memorable characters, on storytelling, and especially on formatting. I am an optioned writer, who has written several scripts (and TV sitcoms), yet I find Trottier's work an invaluable reference.
"The Screenwriter's Bible" is also useful as a marketing tool, especially his sample project plan and action plan, which all writers will find useful as a template. Remember, your great spec script is a business proposal as much as anything else, for a project which will run well into 8 figures, in most cases. Approach it with the same care and attention to detail that you would if you were trying to sell your garage software development firm to Microsoft.
Unlike many "How-to" screenwriting books, Trottier's sample query letters are actually quite entertaining. If I were a Hollywood suit, I'd bite at the sample "Wizard of Oz" example.
This book won't tell you how to write a great story (that's something that really can't be taught), but it will help you get the mechanics of the trade down. There are many terrible scripts which have been made into movies, but virtually all of them (even those starring Pauly Shore) get the details correct.
In summation, if I had to pick just one screenwriting book, this would be my first choice.