The Screenwriter's Workbook: Exercises and Step-by-Step Instruction for Creating a Successful Screenplay (A Dell Trade Paperback)
Quintessential screenwriter's how-to book. I read this a few times and reference it now and then as well. It was a tremendous help in getting me off the ground when I first started writing scripts, especially since I'd never even taken a class in it or had any study on the subject whatsoever.