It’s important to read screenplays, especially in the genre you’re writing. You can often find them for free on the internet.
To read scripts:
http://www.script-o-rama.com/
http://www.simplyscripts.com/
http://www.dailyscript.com
or go to the Writers Guild Foundation Library
http://www.wgfoundation.org/library.aspx
Recommended books:
The Art Of Dramatic Writing: Its Basis In The Creative Interpretation Of Human Motives by Lajos Egri
Story: Substance, Structure, Style and The Principles of Screenwriting by Robert McKee
The Anatomy of Story: 22 Steps to Becoming a Master Storyteller by John Truby
Save The Cat! The Last Book on Screenwriting You’ll Ever Need by Blake Snyder
Writing Screenplays That Sell: The Complete, Step-By-Step Guide for Writing and Selling to by Michael Hauge
A Story is a Promise by Bill Johnson
Bird by Bird by Anne Lamott
On Directing Film by David Mamet
Three Uses of the Knife by David Mamet
On Writing by Stephen King
The War of Art by Steven Pressfield
Recommended publications:
Creative Screenwriting Magazine (also check out their podcast)
Filmmaker Magazine