The most essential screenplay book I have found thus far is definitely
Cole/Haag's *The Copmlete Guide to Standard Script Formats Part I* (Part
II is for television scripts). I find it chock-full of usefull tips. It's
a reference book I keep going back to.
 
Another good book is David Trottier's *The Screenwriter's Bible*. Although
it's not as in-depth as the Cole/Haag book, it covers more ground and a
fun read.
 
Screenwriting professors will tell you the importane of backstory and two
good books for creating that and good characters are Lajos Egri's *The Art
of Dramatic Writing* and *The Art of Creative Writing*. They do not deal
directly with screenplays, but they're good nonetheless.
 
Hope I could be of help
Dani
 
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