Save the cat! : the last book on screenwriting you'll ever need

Have your hero do something sympathetic (such as saving a cat) at the beginning of the film, and the audience will follow him the rest of the way is the eponymous bit of advice in this one. Snyder was a working screenwriter, and his fresh-from-the-trenches advice distinguishes this one from the crowd. A snappy compendium of screenwriting wisdom.