I don't have his Understand Exposure, but I do have his Understand Photography Field Guide that I am currently reading in the Kindle App on my iPhone. I think it works great and the full color photos really help. I couldn't imagine using it on a regular kindle in black and white though, especially since some of this book deals with colors.
I only got the one I did because when I searched in mid December, it showed that it was getting a kindle version in about a week, so I went ahead and pre-ordered it. The Understanding Exposure didn't mention an upcoming Kindle Version. I just now realized that they added that one as well at the same time. Oh well, I find the book very interesting and have learned quite a bit from it so far.
On a side note, the bible according to Lighting, AKA Light: Science and Magic is also available in the Kindle Store. Also with full color photos. I just have the sample version of this one so far until I finish Understanding Photography.