I'm hardly an expert, but these two books really made sense to me.
First is Syd Field's book Screenplay: The Foundations of Screen Writing. Its strength is in how it goes over the structure of a story, explaining the different parts. There are other books that came later, like Save the Cat, but I still like Syd's book more. It's broader.
The other book I really like is Writing Into the Dark by Dean Wesley Smith. He goes into how to write a book without doing a bunch of outlining, how over planning can ruin the fun of writing. It's great.