If you need a book to be able to write a good selling screenplay then you will never sell a screenplay, you sell your screenplay by using your imagination and coming up with a good movie idea, then working hard to write it over and over and over and over until its perfect.
Personally, I agree with you. But some do not have ‘devine intervention’ or ‘muses’ when it comes to writing, and need a little direction. Sure the script may come out like a cardboard box. But with practice and rewrites, you never know what the final draft may be.
I had trouble with structure and had to read about it, the basics, the mechanics of a screenplay. Not the creative aspects, I had that shit down. Basically I had to conform to what they wanted a screenplay to read like. So I switched a few acts and presto, I had structure without compromising an inch of what I had created.
Screenwriters are HIGHLY under rated. A professor once asked me what was the one element a movie could not do without. I said, the screenwriter. The answer was the actor. I was highly pissed. My thoughts were, someone needs to tell those idiots what to say and then I remembered Improv. But Improv isn’t always good, so there.