R C Sproul has written a great commentary here which every serious student of Romans should have on their shelf. It reads more like a set of sermon notes to be honest, which just makes it all the more accessible if you're looking for something you can read through from cover to cover. My only criticism would be that this is very old school Reformed, and so misses the point in my opinion of Romans 6 and 9-11. However a great text all round - maybe read this alongside N T Wright's commentary on Romans and you have the whole story. Highly recommended.