Not being a filmmaker myself, I was surprised at how much I enjoyed this book, and how applicable I found it to my writing. Seger broadens the minds of those who believe "the book is always better than the movie. period." (I used to be one of them) and causes you to view a film adaptation as just that--an adaptation, or translation, from one medium to another. Understanding the essential workings of each medium will help you to appreciate a filmmaker's interpretation; you will learn what makes a story have the potential to "work" as a film, the tools available to a filmmaker, and what choices are necessary to make in order to create a workable film.
This book focuses a lot on "finding the story" in a novel--identifying and understanding theme, character development, plot, and climax, in order to translate it effectively to the screen. I have found this advice most applicable to my child/adolescent writing projects.