If you're looking to getting in to writing some more in-depth screenplays (or just actually starting to write them) then I can't recommend any other book but the Raindance Writers Lab: Write and Sell the Hot Screenplay.

It's got a great quasi textbook-guide style to it that will guide you through the first steps towards making a screenplay (but not so badly that you feel its limiting your creative scope)

Although it has a slight Hollywood bias towards it in terms of the story structure sections, this sort of study is essential to understand where to 'bend' the rules and conventions.

Just thought I'd drop in this nugget of knowledge to ya.