This is a great question and when I read this quote at the beginning of Mississippi Blood, I knew it was coming. Regrettably, I am not qualified to answer it, for I have not yet read this great work by Penn Warren. It has to be said that in quoting Penn Warren and with the passing of Harper Lee, Iles must have been thinking about the work of these two southern writers as he plotted and penned his trilogy. The spelling of Penn Cage's name seems patterned on Penn Warren's, and the characters of Tom and Penn Cage both make readers want to make comparisons with Willie Stark and Atticus Finch. The setting of the third book in the trilogy - almost entirely in the courtroom - seems homage to Harper Lee, and the power struggle apparent in both good and evil forces in the book is, in many ways, reminiscent of Mockingbird - and by what you say in your question to All The King's Men. So while I cannot compare these works yet, it seems to me that Iles is not only asking us to do so, but in his very own way carrying the torch of the two into the new millennium.