Conversations With My Agent
By Rob Long
This one is more for the flavor of life than anything else. Rob Long is a gifted guy, wrote for Cheers (among other things) and did this book back in 1998. I haven't read it in a long time, but I remember being struck both by the wonderful humor of the writing (no surprise) and the nature of his relationship to his agent, believe it or not.
It sounded like a humorous fabrication then. I'm much less certain now.
Seriously, there aren't many good good books about being a screenwriter, but this one is fast and interesting. Check it out.