I've read a lot of books, but I'm still not very good at writing.  
The book that probably helped me the most though was "How NOT to Write a Screenplay" by Denny Martin Flinn. He explains things in a very easy way and he's very funny too.

I believe he's a produced screenwriter and novelist and used to be a studio reader as well. He's very good at explaing what the "reader" want to see and what they chuck in the trash.

I'll post three as well:

The Screenwriter's Bible, by David Trottier.  This one's pretty common knowledge, and for good reason.  Indespensable.

How To Write A Selling Screenplay, by Christopher Keane.  What I like most about this book is that Keane includes a screenplay he wrote and which has been kicking around the studio system for years.  It's a great script, and he goes into detail on why sometimes even the best scripts never make it to a screen -- including his.

I also want to also point out one that isn't a "how to" book, but an actual published screenplay...

The Shawshank Redemption published screenplay, by Frank Darabont with a forward by Stephen King.  I mentioned this in another thread but can't stress enough how truly informative this book is.  It contains the actual screenplay (shooting draft) and includes a section which discusses scenes which were cut from the final film and why.  Darabont gives some invaluable information here.

It's more than three, but I compiled a list some time ago:

The Screenwriter's Bible by David Trottier -- this is my number one most recommended book. I even posted the major points on his screenwriting workbook in the Screenwriting Class under "Screenwriting Palette" -- it's a sticky.

Screenwriting 101: The Essential Craft of Feature Film Writing by Neill D Hicks  

Elements of Style for Screenwriters by Paul Argentini    

How Not to Write a Screenplay: 101 Common Mistakes Most Screenwriters Make by Denny Martin Flinn  

Creating Unforgettable Characters by Linda Seger    

Making a Good Writer Great: A Creativity Workbook for Screenwriters by Linda Seger  

The Tools of Screenwriting: A Writer's Guide to the Craft and the Elements of a Screenplay by David Howard, Edward Mabley (Contributor), Frank Daniel (Introduction)    

Secrets of Screenplay Structure: How to Recognize and Emulate the Structural Frameworks of Great Films by Linda J. Cowgill  

Selling a Screenplay: The Screenwriter's Guide to Hollywood by Syd Field    

How to Write a Selling Screenplay: A Step-By-Step Approach to Developing Your Story and Writing Your Screenplay by One of Today's Most Successful Screenwriters and Teachers by Christopher Keane, Julius Epstein (Foreword)  

500 Ways to Beat the Hollywood Script Reader : Writing the Screenplay the Reader Will Recommend by Jennifer Lerch    

You Can Write a Movie (You Can) by Pamela Wallace  

Complete Idiot's Guide to Screenwriting by Skip Press    

Afi Writing Great Screenplays for Film and TV (Writing Great Screenplays for Film and Tv, 2nd Ed) by Dona Cooper  

The Elements of Screenwriting by Irwin R Blacker    

Laughing Out Loud: Writing the Comedy-Centered Screenplay by Andrew Horton  

Writing Treatments That Sell: How to Create and Market Your Story Ideas to the Motion Picture and TV Industry by Chi-Li Wong, Kenneth J. Atchity    

Writing the Second Act: Building Conflict and Tension in Your Film Script by Michael Halperin  

Writer's Guide to Hollywood Producers, Directors, and Screenwriter's Agents, 2002-2003: Who They Are! What They Want! And How to Win Them Over! by Skip Press    

The Perfect Pitch: How to Sell Yourself and Your Movie Idea to Hollywood by Ken Rotcop, James K. Shea, Marlane McGarry (Editor)  

The Writer Got Screwed (but didn't have to) : Guide to the Legal and Business Practices of Writing for the Entertainment Industry by Brooke A Wharton    

Story: Substance, Structure, Style and The Principles of Screenwriting by Robert McKee  

I found this list online some time ago, and held on to it, planning to get at least some of these books. I want How To beat The Hollywood Reader to satisfy my curiosity there. It's supposed to be written by a reader.

I've got another that hasn't been mentioned.  It's called "Filmmaker's Dictionary" by Ralph S. Singleton and James A. Conrad.  It's 350+ pages of terms for both film and script.  Very useful.  

There's only one screenwriting book I actually bought and that's Teach Yourself: Screenwriting. Most of the entire book is about the prewriting process but it has good advice on sending it out once it's finished too. I really like it. But I've read Screenwriting for Dummies and The Complete Idiot's Guide to Screenwriting and those are good too.

Story: Substance, Structure, Style and The Principles of Screenwriting by Robert McKee  

This is the only book I've ever read about screenwriting and it taught me a hell of a lot. The first thing it taught was that I knew bugger about screenwriting and after I read it I knew a lot more.

I also attended McKee's screenwriting seminar when he was in Melbourne and learnt a lot from that too.

Here's McKee's Ten Commandments

1 Thou shalt not take the crisis/climax out of the protagonist's hands. The anti-deux ex machina commandment.

2 Thou shalt not make life easy for the protagonist. Nothing progresses in a story, except through conflict.

3 Thou shalt not give exposition for strictly exposition's sake. Dramatize it. Convert exposition to ammunition. Use it to turn the ending of a scene, to further the conflict.

4 Thou shalt not use false mystery or cheap surprise. Don't conceal anything important that the protagonist KNOWS. Keep us in step with the hero. We know what he/she knows.

5 Thou shalt respect your audience. The anit-hack commandment.

6 Thou shalt know your world as God knows this one. The pro-research commandment.

7 Thou shalt not complicate when complexity is better. Don't multiply the complications on one level. Use all three; Intra-Personal, Inter-Personal, Extra Personal.

8 Thou shalt seek the end of the line, the negation of the negation, taking characters to the farthest reaches and depth of conflict imaginable within the story's own realm of probability.

9 Thou shalt not write on the nose. Put a subtext under every text.

10 Thou shalt rewrite.

I've also found McKee's "Story" invaluable, though I know some find him too bombastic.  

I don't think "The Writer's Journey" by Christopher Vogler has been mentioned .  It's about the relationship between film stories and myth and how mythic archetypes inform modern storytelling.  Not a "tips" book, but a fascinating read if you are unfamiliar with the whole Joseph Cambell la-di-da.

Quoted from Hecate I've also found McKee's "Story" invaluable, though I know some find him too bombastic.

I liked him. When I spoke to him during the breaks at the seminar, he came across as a very serious but reasonable guy. Anyway, he’s taught me a hell of a lot about screenwriting; I have nothing but respect for the man.

It seems quite a few people like the Screenwriter's Bible, I've added that to my list

"I don't think "The Writer's Journey" by Christopher Vogler has been mentioned .  It's about the relationship between film stories and myth and how mythic archetypes inform modern storytelling.  Not a "tips" book, but a fascinating read if you are unfamiliar with the whole Joseph Cambell la-di-da."

Thats right, "The Writer's Journey" by Christopher Vogler  is one of the best!

By the way this book is Kevan R Craft's favourite one!

Quoted from Takeshi It seems quite a few people like the Screenwriter's Bible, I've added that to my list

Me too - I'd never heard of it before!  

Has anyone tried "Screenplay" by Syd Field?  I got a couple of chapters in and gave up.  He seemed very prosaic and plodding compared to McKee (not to mention dated...)

Quoted from Hecate Has anyone tried "Screenplay" by Syd Field?  I got a couple of chapters in and gave up.  He seemed very prosaic and plodding compared to McKee (not to mention dated...)

I had a copy of it about ten years ago but lost interest. All I really remember is him waffling on about the paradigm.

I must be the only one not to have read ANY books on screenwriting. I did learn by myself, only reading existing scripts, watching movies and reading subtitles. I never wanted to read books on screenwriting because they're generally never written by screenwriters or people who write on movies. If there was any good recipe to write the perfect screenplay, nobody would be here. We'd all meet in Hollywood.

My secret is just WORK, WORK and WORK again. And most of all, never being discouraged.

Michel

Quoted from Hecate Has anyone tried "Screenplay" by Syd Field?  I got a couple of chapters in and gave up.  He seemed very prosaic and plodding compared to McKee (not to mention dated...)

This is the book which kicked off my interest in screenwriting.  An old roommate picked it up and said something along the lines of hey, check this out, it says how to write a movie.  So, being a film buff I gave it a read.  Found that it read quite a bit like stereo instructions but it got me interested nevertheless.

The thing with screenwriting books is that they're okay to a point, but at the end of the day the most important thing a writer can do is sit their ass down and write, that's the true discipline.

Quoted from Takeshi The thing with screenwriting books is that they're okay to a point, but at the end of the day the most important thing a writer can do is sit their ass down and write, that's the true discipline.

I agree 100%.
I have a huge problem with the self discipline required to actually finish something.

Another thing is that people can read about how to do things until they are blue in the face. Knowing all the technical and structural aspects of writing, but ultimately, it always comes down to the story itself. I have read scripts that were not quite perfect technically, but I didn't care, I just kept reading because the story itself was that good. I've also read scripts that were spot on with no problems what so ever regarding the tech part, but I could barely finish it because it bored me to death.