The Pirenne thesis is further elaborated in this dense and somewhat antiquated but still valuable and insightful book
First coming to prominence in his monumental Mohammed and Charlemagne, Pirenne here expands and defends his notion of the decline of antiquity and the creation of a Medieval society. No greater summary of the Pirenne thesis can be given than by Henri Pirenne himself in the introduction to this book:
"the barbarian kingdoms, founded in the fifth century in the soil of Western Europe, still preserved the most striking and essential characteristics of ancient civilisation, to wit, its Mediterranean character...
...It was only the abrupt entry of Islam on the scene, in the course of the seventh century, and the conquest of the eastern, southern and western shores of the great European lake, which altered the position, with consequences which were to influence the whole course of subsequent history. Henceforth...the age old link...became a barrier." (p.1-2)
Pirenne's ever pertinent thesis, which flies in the face of our modern western obsession with self-blame for economic success, and the Marxist notion that all wealth must be ill-gotten, is that the centre of gravity in Europe shifted North to Paris following the Islamic invasions (and the collapse of the Merovingians who were not stagnant dark-aged lords but the last of classical antiquity). This was an economically disasterous shift but gradually European prosperity re-emerged as trade once again expanded, in the new Northern cities following the defeat of the Vikings, and in the Mediterrenean undet the umbrella of the Byzantine and as Europe re-flexed its military might in the Mediterranean with the Crusades and Italian city states (along with the Catalan traders) gradually displaced the Saracene pirates, raiders and slave-traders. Though this threat to European trade would not be eliminated until the Barbary pirates were themselves eliminated centuries later.
This trade would bring about an increase in wealth (and a decrease in slave raiding) allowing the European ecoonomy to support the growing cities, creating a self-perpetuating economic growth and the cities further developed trade and wealth.
It should be noted though that this book, despite the claim to Medieval European history, only briefly mentions the Byzantine Empire and only mentions it in relation to the Western Carolingian Empire.
To gather a sense of the book, the table of contents is helpful:
Chapter 1: The Revival of Commerce
I. The Mediterranean
II. The North Sea and the Baltic
III. The Revival of Commerce
Chapter 2: The Towns
I. The Revival of Urban Life
II. Merchants and the Bourgeoisie
III. Urban Institutions and Law
Chapter 3: Land and the Rural Classes
I. Manorial Organisation and Serfdom
II. Changes in Agriculture from the Beginning of the Twelfth Century
Chapter 4: Commerce to the End of the Thirteenth Century
I. The Movement of Trade
II. The Fairs
III. Money
IV. Credit and the Traffic in Money
Chapter 5: International Trade to the End of the Thirteenth Century
I. Commodities and Directions of International Trade
II. The Capitalistic Character of International Trade
Chapter 6: Urban Economy and the Regulation of Industry
I. The Towns as Economic Centres. The Provisioning of Towns
II. Urban Industry
Chapter 7: The Economic Changes of the Fourteenth and Fifteenth Centuries
I. Catastrophies and Social Disturbances
II. Protectionism, Capitalism and Mercantilism
Pirenne tries to balance both the role of the Church and Capitalism (if you want to know his bias, I can't tell from the book, they are like the brake and accelerator of the Medieval economy, one protecting the weak, the other rising the general standard of living) and neither are above his often humouristic reproach:
"Thus, the Church was not only the great moral authority of the age, but also the great financial power." (p. 13)
"The Church itself was continually obliged to borrow from the financiers whose actions it reproved." (p. 138.9)
"[Capitalism] corresponds with man's acquisitive instinct." (p. 162)
"these capitalists, for the most part, sprang form the dregs of society, déracinés, who as soon as trade revivedtoo to it with no assets but their energy and intelligence, their love of adventure and no doubt also their lack of scruples." (p. 162)
Overall, it is a great but frequently long and heavy read.