Adventures in the Screen Trade, by William Goldman, is a fascinating look at what being a mainstream hollywood screenwriter is about. I must confess that while I consider myself to have finished it, I've probably only read about 2/3 of it. Written in an extremely personal, conversational style, the portions of it which recount Goldman's experiences are extremely interesting and education, but like C.S. Lewis's The Screwtape Letter, it's difficult reading. (For those who haven't read that great work of Lewis's, the words are easy, but the emotions are tiring.)
If you're interested at all in mainstream screen writing, or in how the plots of mainstream hollywood movies are made, I heartily recommend it. The plots of mainstream movies are created with a process that only starts with the screenwriter, and Goldman covers this in great depth. Like most human activities, film making is really an ad-hoc affair; a large number of people scramble to get it done and fulfill their own goals as well as they can, and the dynamics of this are varied but Goldman makes them seem understandable.
I was drawn to the book by my interest in screenwriting (I haven't done any yet, but I am quite interested in it) and am quite glad that I bought the book.