BEST SCREENWRITING BOOKS
The definitive text in the field, by Syd Field.
Writing a Great Movie
Your movie could be as great as this book - by Jeff Kitchen
Devil's Guide to Hollywood
If your script reads as fast as this book, you're doing it right - by Joe Eszterhaus
Save the Cat
Crafting the structure of a saleable Hollywood screenplay - by Blake Snyder.
On Directing Film
David Mamet lays it down tersely, with minimal exposition - by David Mamet.
Secrets of Writing For Film
An excellent source of techniques by the writer of Stigmata - Tom Lazarus
The Hollywood Standard
This covers the Warner Bros style formatting, which is what I prefer and recommend. Some people are against this format for spec screenplays and suggest that you write as if you were a disposable rather than essential element of moviemaking.
SCREENWRITING BOOKS YOU MAY FIND USEFUL
Anatomy of Story
You might find this useful. I found it a tedious read. - by John Truby
A short and useful book about the history reels and sequences. Informative.
How to Write a Screenplay in 21 Days
One or two helpful ideas.
AVOID THESE SCREENWRITING BOOKS
If you're having trouble with insomnia, this book will put you right to sleep.
How not to write a screenplay
How not to write a screenplay book.
The Only Screenwriting Book
With chapters copied directly from Syd Field's book. Not recommended.