In this fine book Florian Mazel, professor at the University of Rennes-2, takes issue with the dominant scholarly assumption that the shape and nature of dioceses in the late Roman Empire persisted across Continental Europe until fairly recent times (in France until the Revolution), with Christian bishops simply replacing Roman civil adminstrators. The author challenges what he sees as an untested assumption of continuity, calling forth numerous examples of disruption and discontinuity.
Mazel examines in great detail how over the period from the fifth to the tenth century the bishop consolidated power in his city, guiding its development from a civitas to a cité. The link between the bishop and his city was exclusive and definitive: exclusive because there were few rival powers, definitive because the bishop was not to relocate outside his mother church (he was even expected to be buried in or near his cathedral). Further, the bishop was in charge of urban defense, resolving local social issues, and consecrating churches. He reinforced his power through liturgy, rituals, processions, and relic cults.
In the eleventh century that power began to to radiate outward from the city. The bishop sought to regain control over monasteries heretofore independent of episcopal power. He asserted influence over peripheral areas contested by neighboring bishops. He further expanded his power via more constant visitation of outlying churches, liturgical processions linking the countryside with the city, and especially by use of diocesan synods which brought outlying clergy to the cathedral church on an increasingly regular basis.The diocese, now under full control of its bishop, gained greater clarity both in terms of geography and terminology: for example, the word episcopatus gradually superceded parochia as the area controlled by the bishop.
By the end of the twelfth century the bishop had further consolidated his power. He had increased episcopal dues and gained additional authority over exempt monasteries, as well as over parish priests and parish churches.Terminologically, diocesis gradually replaced episcopatus. By the end of the thirteenth century these trends had continued with bishops overseeing subordinates such as archdeacons, having access to increased revenues and properties, and drawing on systematic records to prove and secure that wealth. Moreover, these powers were increasingly recognized by papal confirmations.
The concentration of episcopal power, however, is only part of Mazel's story. For he is also interested in the medieval concept of territory and how it differed from Roman one. Following Max Weber, Mazel holds that territory is essentially the power of an institution projected in spatial terms, the way power can define territory. Unlike scholars who assume that the medieval church merely borrowed its understanding of territory from the Romans (especially in the configuration of the diocese), Mazel argues that the power of the bishop was not expressed in a homogeneous fashion within a clearly defined spatial territory, but was instead heterogeneous, based on varied personal ties which united people and place. Mazel is fond of the term un maillage territorial, an administrative network in which subordinates interweave personal service to the bishop with spatial domination, the best example being the rise first of archdeacons and then of their delimited archdeaconries. Mazel does not express it this way, but one could say that the Roman understanding of administrative space such as a diocese was imposed from above, while the medieval understanding was built from the ground up, as the administrator in charge of the diocese (the bishop) cemented personal ties outward over time, eventually arriving at a robust administrative hierarchy, leading in turn to clearly defined borders. Moreover, Mazel reminds us that this new, diocesan notion of territory was passed on to the state in the second half of the thirteenth century, especially in France.
We have not yet emphasized the abundance of historical evidence that the Mazel's arguments rest upon. Given his previous works on medieval France it is perhaps not surprising that the bulk of his examples are French, but he does not ignore Italy or the Holy Roman Empire, and even has the occasional example from Spain. Certainly, his discussion of England could have been beefed up, given that it, too, supports the generalizations that the author wishes to make.  Perhaps the only major deficiency of the work is the failure to consider relevant theoretical perspectives. Mazel does include a brief historiographic discussion (160-163), but this has to do with the blind acceptance of modern geographers of Roman continuity. It is surprising, given the book's subtitle, that more orientation is not provided on the so-called spatial turn. Indeed, a more accurate title would be L'Évêque et son territoire: Une contribution au discours sur l'invention médiévale de l'espace (Ve-XIIIe siècle).  Mazel certainly incorporates recent spatial theory throughout the book, but for something along the lines of a "state of the question," one must look elsewhere. 
Finally, the book is bolstered by numerous maps, charts, and indices, as well as endnotes, and a comprehensive bibliography. On the whole, I found Mazel's work challenging, clear, interesting, and ultimately convincing.
1. Thus, Frank Barlow, The English Church 1000-1066: A History of the Later Anglo-Saxon Church (2nd edn; London: Longman, 1979), passim; Barlow, The English Church 1066-1154: A History of the Anglo-Norman Church (London; Longman, 1979), chapters I and III; Martin Brett, The English Church under Henry I (London: Oxford University Press, 1975): "It is not until the end of the thirteenth century that it is possible to draw a complete map of the dioceses of England" (101).
2. Oddly, the qualifier (Ve-XIIIe siècle) appears on the book's front cover, but not on its official title page.
3. See, recently, Meredith Cohen and Fanny Madeline (eds.), Space in the Medieval West: Places, Territories, and Imagined Geographies (Aldershot, Surrey: Ashgate, 2014), 1-17.