This is a major book, a rich, dense, and important study of both the mature corporative fascism of the Regime (chs. 8-12), and more importantly, of its evolution -- or rather, the evolution of one part of it (for the Nationalists, like Alfredo Rocco, while discussed, are not the major focus of Roberts' study) -- out of the revisionist Marxism of the Italian Syndicalists, who eventually came to replace class with will (that is, materialism with voluntarism).
This is a brilliant book -- insightful, dense, and analytical -- and is an essential complement to A. James Gregor's even more brilliant (and more flawless) Young Mussolini.
Highly recommended.