After watching Adaptation you should have bought the book "Story" as mentioned earlier.

I think my favorite book on screenwriting is Successful Scriptwriting. I also enjoy Zen and the Art of Screenwriting which is a collection of interviews with top screenwriters. The interviews are often very insightful and interestingly, every screenwriter seems to have their own take on writing and the idea of learning the craft of screenwriting. I highly recommend this book.

writing a script...
 

I've written several... I can tell you the best path is to read screenplays... if you want to write the quintessential action movie, read DIE HARD a couple times, which is to say you have to KNOW your genre. Don't try and write a romantic comedy if you don't understand the genre rules of romantic comedy. If you want to learn rules, go to Aristotles POETICS or Lajos Egri's THE ART OF DRAMATIC STORYTELLING.

It's equally if not more useful to read some scripts of movies you haven't seen. If you're making your own scripts, format isn't that important; if you're selling scripts, that's a different story. But read scripts, and don't waste your time reading books about writing scripts, in my humble opinion (other than the 2 mentioned above). Those works are highly intellectual research papers into screenwriting.

theturnaround

Screenwriting 1...2...3...
 

Assuming that you are familiar with how to format a screenplay (spacing, caps, log lines,etc.) I suggest you start with a book called "How To Write A Movie In 21 Days", by Viki King. If you do what the book says you'll be able to knock out your first screenplay in no time. Once you've written it, File it away! Then go buy Syd Fields Book. Master his technique. After you have mastered his techniques and are comfortable with his method, rewrite your old screenplay using what you've learned. Once again, don't go out and get an agent just yet. Pick up a book Called "Story" by McKee. Read his book over and over untill you hear him talking to you like Obe One Kanobe as you write your screenplays. By this time you should be ready. Now rewrite your screenplay once more using the "Jedi" Force of Fields and McKee as you write. By then you will be ready "Grasshopper".