Read Yourself says Cason Reily. I read but there were several points not too clear and reviews helped. Off and on he seems to use neoclassical economics and whether these confirm his data is not clear to me. Right now there is a debate going on about his second law. I found Seth Ackerman's review (and summary by Steve Roth) interesting in this respect. He believes the data, doubts the models (not even wrong) and reinterprets Piketty's conclusions. That is as far as I got and am still struggling to understand the significance of the second law and how it is used. At the moment, the explanation of Dan Kervick (the first comment in Brad DeLong's post) seems fine to me.