Whether the conversation related to screenwriting, directing, or any other film discipline, one of the most common questions I get is most fertile tofledgling filmmaker.
There's a few books I always recommend:
The Screenwriter's Workbook, by Syd Field
Making Movies, by Sid Lumet
An Actor Prepares, by Stanislavski
At the same time, the one person who has given me the most raw, useable information about the art of cinema is Roger Ebert. Paradoxically, Ebert never comes up when people talk about great places to go to for information on film!
Who in history has been more knowledgeable about movies? Name one observer of cinema with more experience or insight. Fortunately for filmmakers, Ebert writes for a living! Why Roger Ebert is not at the top of everyone's reading list, I'll never fathom.
One objection I hear a lot is that his taste in film differs from mine, or yours, or anyone's... Really, I don't care whether Ebert likes or dislikes a given movie. His reasons for doing so are where his acute sense of cinema shines through, and all his energy goes into illistrating those points as completely and clearly as one can in a few hundred words.
For pure cinematic eye-opening power, Roger Ebert is the best use of your time.