Fitzempress' Law
This is an excellent book. I can't imagine why it is out of print. I read it about 15 years ago in a library, and have been looking for a copy of my own ever since. I even offered to buy it from the library, but they wouldn't consider it.
The author has a marvelous knack for saying things in a memorable way. I have used her quotes when teaching writing at conferences.
I have read other books by Diana Norman and didn't care for them, (they seemed lightweight and porno, to me) but this one is great.