Although I have over 100 books on the subject, on my desk, I have and use:
1. The Screenwriter's Bible: A Complete Guide to Writing, Formatting, and Selling Your Script by David Trottier
2. The Chicago Manual of Style: The Essential Guide for Writers, Editors, and Publishers (14th Edition)
3. Chris Vogler's THE WRITER'S JOURNEY
4. Joseph Campbell's The Hero with a Thousand Faces
5. The 101 Habits of Highly Successful Screenwriters: Insider's Secrets from Hollywood's Top Writers by Karl Iglesias
6. McKee's Story
7. Blueprint for Screenwriting: A Complete Writer's Guide to Story Structure and Character Development by Rachel, Ph.D. Ballon, Rachel Friedman Ballon
8. The Screenwriter's Workbook by SYD FIELD
9. How To Write High Structure, High Concept Movies by Rob Tobin
Normally I wouldn't shamelessly plug a friend's work, but a member of my online screenwriters networking group, The Screenplayers (
http://www.screenplayers.net), just published a pretty good book called:
Save The Cat! The Last Book on Screenwriting You'll Ever Need by Blake Snyder
I read it (he was kind to send us all a copy with many thanks) and it deals alot with the new 4Q screenwriting theory which enhances the 3 act structure. It's easy to read, fun and gives good insights to a new writer.