Screenwriting is NOT formulaic tripe as so many of the modern books emphasise. There is no structure that will give you certain success, nor is there an outline to great writing that you can cut and paste and reap millions. most writers make their living carving out a small niche for themselves while ensuring that their name carries the weight of good and solid work behind it.
The only book you need to read is the one that is outside your door. the stories that exist around you. the trials of humanity, the foiled love, the harsh words of a parent. THESE are the emotions that people want on screen and stage. Not the boiled down rubbish that most film books tell you to read.
But on the other hand life is as complex as a snowflake, and self taught geniuses are at an all time low. The following list is a guide not for what you SHOULD write but the nature of the game that you are entering and the paths that you should try and conquer to get the success that you want.
www.script-o-rama.com one of the easiest and most prolific of the screenwriter’s resources. It has over two thousand screenplays. More important than the books on screen writing are the book from screenwriters.
William Goldman’s screen writing book. from one of the best scribes in Hollywood comes one of the most interesting and knowledgeable books on the nature of the beast.
These two resources along with the Internet’s massive amount of data will enable you to start on your road to writing without shelling out teh precious dllars you could use to but more paper and ink! Take care, and Write well!