Thomas F. X. Noble 
The Republic of St. Peter [PDF ebook] 
The Birth of the Papal State, 68-825

The Republic of St. Peter seeks to reclaim for central Italy an important part of its own history. Noble’s thesis is at once original and controversial: that the Republic, an independent political entity, was in existence by the 730s and was not a creation of the Franks in the 750s.
Noble examines the political, economic, and religious problems that impelled the central Italians—and a succession of resolute popes—to seek emancipation from the Byzantine Empire. He delineates the social structures and historical traditions that produced a distinctive political society, describes the complete governmental apparatus of the Republic, and provides a comprehensive assessment of the Franco-papal alliance.