Uta-Renate Blumenthal 
The Investiture Controversy [PDF ebook] 
Church and Monarchy from the Ninth to the Twelfth Century

‘This book describes the roots of a set of ideals that effected a radical transformation of eleventh-century European society that led to the confrontation between church and monarchy known as the investiture struggle or Gregorian reform. Ideas cannot be divorced from reality, especially not in the Middle Ages. I present them, therefore, in their contemporary political, social, and cultural context.’—from the Preface