A major problem which occupied thinkers in the later Middle Ages was the question of the internal structure of the Church and the proper interrelationship of its members. Dr Tierney's book is an account of those canonistic theories of Church government which contributed to the growth of the conciliar theory, and which were formulated between Gratian's Decretum (c. 1140) and the Great Schism (1378). It is concerned particularly with the juristic development of the fundamental conciliar doctrine, the assertion that the universal Church was superior to the Church of Rome, with a consequent denial of the Pope's supreme authority.
This will probably be the most exciting book that you will ever read about corporate law! Brian Tierney traces the development of conciliar theory - that the power of the Catholic Church is derived from the universal body of the faithful rather than from the pope alone - from its earliest roots in 12th century canon law. That sounds rather dry, but it's honestly a fascinating read that delves into the problem of exactly where authority rests within the medieval church, and how the different bodies within the church relate to each other.
Tierney's main point is that conciliarism isn't an external imposition on canon law, the quasi-heretical ideas of Marsilius of Padua or William of Ockham superimposed onto a solid foundation of papal monarchy. And it's not simply a translation of conceptions of constitutional monarchies that were developing in the emerging secular nations around the same time. Instead, it's the natural culmination of 200 years of canon law development that resulted when the ideas of the first Decretists joined with the concepts of corporate law explore by their successors.
Tierney writes exceptionally clearly and compellingly, and this is a must read for anyone who is interested in the medieval church or in the Catholic Church of the present day.