Portraiture Unplugged by Carl Caylor

Carl talks about the basics of photography and how he built his natural light studio. Then he provides numerous samples of his work both indoors and out. Each image's lighting is explained as to why he did what he did and sometimes how he would have improved upon the image. He also has a diagram with each picture which I always like. It's a good book to help push your natural lighting skills further.
Find it here:
Amherst Media
Amazon
Barnes & Noble