A friend of my wife's is taking a film photography class and I guess the professor is not that good.
She wants a book on understanding exposure.

Does anyone have any recommendations? She is a completely new to photography I might add.

You already told yourself the answer

Understanding Exposure by Bryan Peterson is the "bible" of - you guessed it - understanding exposure. It was one of the first books I bought almost a decade ago and the only one I've never contemplated selling, even though I now know it backwards and forwards.

+1 for Understanding Exposure - This book is excellent!!!! Bryan Peterson explains the photographic triangle; aperture, shutter speed & iso with clarity and ease of understanding. A must have book.

Bryan Peterson's most recent (just released on March 31st) book is called 'Understanding Shutter Speed'. I ordered this book from Amazon and can't wait to read it. If it's anything like 'Understanding Exposure' it will become another "must have" photography book.

I'm an idiot...I have that book. I bought it a couple of years ago and forgot about it.
thanks guys!

The best book I ever read on exposure was Ansel Adams's The Negative.