It depends on where you are at in your process. Are you a beginner, have you written one or two scripts? Have you sold anything?
Good screenwriting comes from the assimilation of everything that you have read and seen. And that list should include reading as many good screenplays that you can get your hands on. I learned from the bad ones, too, but not at first. At first, you must read good screenplays that have already been produced to get the rhythm, the tone, the pace, and the structure of a saleable script. Saleable being the operative phrase. In reading good screenplays you learn to be economical and smart and bust your own clichés’.
As far as books on writing screenplays, I have read a lot. I take some of the points that make sense and put them in the hopper of my writer’s mind and leave most behind if they confuse me. It helps to be working on a project when you read a book on screenwriting to apply the tips you read to your own project. This personalizes the experience for you and you are more likely to remember it. Since you don’t know what will stick as a beginner screenwriter, read as much as you can, and then let it all go. Trust that the cream will rise to the top.
Personally, I love Syd Field and applied his formula to my beginning scripts. But it really isn’t just Syd’s formula anymore – it is mine and Syd’s. The way I write is my style shaped by reading Screenplay and everything else that I have read, then letting it go. If you write often enough, eventually your instincts and intuitions and life experience will start forming into “screenplay speak”. You will start to see and hear in terms of screenplay.
I did take the infamous Robert McKee’s class but can’t remember a thing except seeing Casablanca and I think he sang on the last day. I love the memoir-type books on writing from the masters: Neil Simon’s Rewrites comes to mind, William Goldman’s Adventure In The Screen Trade, Sidney Lumet’s Making Movies. It’s always fun to learn a successful filmmaker’s process from the inside out.
If you’re just starting out or well on your way, see as many good movies as you can on a regular basis. When I first started out, I saw a movie a day! But it was my passion and the best experience I can remember. Train your mind to think like a selling screenwriter and you’ll become one!