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.