First of all, this book contains no secrets about screenwriting.
Second, it contains no information on how to write screenplays.
The book contains biographical summaries of how a number of screenwriters broke into screenwriting (told in narrative and in interview form), bits and pieces about writing habits, a summary of the movie, and a scene from the screenplay.
The screenplays chosen include six mediocre movies and four fair to good movies.
Studying mediocre movies can be useful provided you ask the all-important questions: (1) Why was this movie mediocre? (2) How could it have been made better? Since these screenwriters don't have a clue that their screenplays are mediocre, they never ask these two questions. (Actually, a number of them admit that their scripts were ruined by studios and rewrites by other screenwriters, but they still don't go into detail as to why those changes hurt the movie.)
The biographies also are depressing: most got their break because they had a friend who was already in or because they had a friend who knew somebody who was already in. Not very encouraging to screenwriters who don't have such connections.
Bottom line: this book is largely a waste of money. Buy a book on screenwriting to learn screenwriting and a book on writing treatments or outlines to learn about submitting material.