Walter Ullmann 
A Short History of the Papacy in the Middle Ages [PDF ebook] 

This classic text outlines the development of the Papacy as an institution in the Middle Ages. With profound knowledge, insight and sophistication, Walter Ullmann traces the course of papal history from the late Roman Empire to its eventual decline in the Renaissance.

The focus of this survey is on the institution and the idea of papacy rather than individual figures, recognizing the shaping power of the popes” roles that made them outstanding personalities. The transpersonal idea, Ullmann argues, sprang from Christianity itself and led to the Papacy as an institution sui generis.