Hhhmmm....I'm a bit surprised. There are a number of good references for this that no one has mentioned.
First off, if you read nothing else, read Scott McCloud's Understanding Comics. It's in a graphic novel format, but nothing will give you a better from-the-ground-up education on visual story telling within the medium of comics. It will also undo a lot of the damage that screen writing techniques will do to you.
If you're feeling good after that, add his Making Comics and Reinventing Comics. Both good information.
Will Eisner is next on the list with his Comics and Sequential Art and Graphic Storytelling and Visual Narrative. Both books are considered the foundation of everything that is done in comics today, and the awards for comics are still called the Eisners. Strongly recommended.
Then there are the books written for writers by writers. The top ones IMNSHO are Peter David's Writing for Comics, and Alan Moore's Writing for Comics, yes, both books have the same title, but are worlds apart in advice and techniques, just as both writers are.
Bear in mind that comic writing is not screen play writing, and while a lot of the underlying principles are the same, just as they are for any other form of fiction writing, comics have unique ways of dealing with things such as time, pacing, and the visual language of comics, so you need references for these things.
Eagle
(I'm a writer first)