These are some of the sources I turn to when I have no idea what the hell I'm doing-
'Three Uses of the Knife'- David Mamet
As he usually does, cuts to the bone, no bullshit.
'Naked Playwriting' by William Missouri Downs and Robin U. Russin
Great book on not only playwriting, but writing/storytelling in general.
'your screenplay SUCKS!' by William M. Akers
It's kind of true, most of the time. Meant for 'mainstream' Hollywood screenplays, it's best used for revising after you've written your beautiful screenplay, and then need to see it in the light of day.
Also, if you ever get the chance, hear Oscar-winning screenwriter (Little Miss Sunshine) Michael Arndt's talk 'Endings: The Good, The Bad, & The Insanely Great', check it out- it's (insanely)
great. There aren't any official copies/videos of his talk, which means there are lots of honorable writers out there, since he makes you promise not to show/tape it (given he reveals some of Pixar's semi-secret early storyboards).