i'm going to go ahead and say that i didn't like Understanding Exposure.
It may very well be that by the time I read it, I had learned so much off the internet that it didn't add much for me, but I also hate the feeling that all he talks about is how much he made off each of his stock photos.
This may also sound a bit backwards, but I learned a lot about natural light photography from the strobist site. Even though that site is all about flash which is the opposite, light is light... and learning the concepts helps whether you're using flashes or just the sun...