Save the Cat is the best screenwriting book I have ever read. Blake Snyder is a master teacher. Getting rid of the usual genre types he breaks movies into new categories that make it easier to determine what kind of movie you are writing and films of the same type. He also teaches you tips and tricks for keeping the audience interested and making sure you don't overload your script with too many plots. My favorite thing was the beat sheet. A different way to outline your script that will change the way you write. Check out his other books as well, Save the Cat Strikes Back and Save the Cat Goes to the Movies. All three will change the way you write and think about movies.
This ultimate insider's guide reveals the secrets that none dare admit, told by a show biz veteran who's proven that you can sell your script if you can save the cat!