yet people think theat the science (or art) of screenplays is merely a couple of books and some minor practice...
Yet it's Randall telling us:
I respectfully disagree. I think it takes more training (books, courses, or both) than what Mamet says to master writing a screenplay.
And I don't consider continuously writing screenplays to be "minor" practice. Writing a full-length screenplay is a lot of work, and you don't just write it once then put it away.
I'm telling you to save your money. Read a script that was turned into a film, watch that film, and read this note from Mamet and you'll have a great start. You don't need to go wasting your time and money on books and courses galore. Those mostly seem to serve as a way to let people excuse themselves from doing actual writing anyway.