There are only two books so enjoyable I've never wanted them to end, Alan Jay Lerner's autobiography "The Street where I live", and this.
William Goldman was screenwriter on "A Bridge too far", "Princess Bride", "Butch Cassidy", "All the President's men" and "Marathon Man". Apropos this is a witty, observant and very readable bible on the art of screenwriting, with it's most oft quoted line being
"Nobody knows anything...... Not one person in the entire motion picture field knows for a certainty what's going to work.
Every time out it's a guess and, if you're lucky, an educated one."
It deconstructs brilliantly the complex and necessarily reductive process of adapting a play/book/idea to the visual medium of the big screen. He explains how what works on the page will NOT work on screen, and how backstories and exposition can be subtly added by glances, actions and scenery.
The author's style is joyfully self deprecatory, with a wonderful hint of Borsch-belt witticisms. Buy it, read it and proudly mount on your bookshelf.