I've just started reading William Goldman's Adventures in the Screen Trade, I'm probably behind every self respecting screen writer on that front, but if you haven't read it you need to right away.
I'm only a hundred pages in, and don't want to quote huge passages from the book, you should go out and buy it but two bits that have stuck out so far are when Goldman discusses the lonliness of the writer, "most of us are entering uncharted ground, and we have hope, but we also know the odds against us. Failure keeps us company."
And also the great section on agents, and the importance of a good query letter.
What a great screen writer, and what a great book. an inspiration to us all.