Learning to write a screenplay is difficult, but finding a good story to write the screenplay is very difficult, but selling the screenplay is harder than both.
Did ya know this author only made one movie that was cute and fun, but did NOT make a lot of money... I give it to you like this... "Those who can't... Teach or Writer a book about it." He did technically make it once, but he could have sold other scripts, but they never got made... why?! So, for me, it's hard to pay attention or take seriously an author of a screenplay book if they dolt have significant credits to show success. Like Aaron Sorkin! I'd read his book in a heartbeat! I have watched his Master class on RU-vid, yes!