First, before you proceed any further with this book, you ought to know that it is not abou the origins of the European economy. If you are looking for a book about economic life and change in Europe between 300 and 900, this is not really the book for you. McCormick's book is specifically about trade, and largely "international" trade, between these years.
Having said that, McCormick's book is the most brilliant work on medieval history in years. He sets out to examine the patterns of Mediterranean commerce during the early middle ages, focusing on different aspects of the Pirenne thesis. This, of course, has been done repeatedly over the eight decades since Pirenne's famous publications, but McCormick's approach is startlingly new. Rather than simply argue over the same tired scraps of evidence, McCormick works hard to incorporate old, non-economic, data into his argument, and also brings in entirely new evidence. To begin with, McCormick focuses on the accounts of non-commercial travellers -- pilgrims, envoys, missionaries, etc. -- to see how they travelled, when they travelled, and whom they travelled with. By looking at these accounts McCormick puts together a picture of frequent Mediterranean travel, demonstrates the frequency of specific routes, and, the interaction of travelling merchants and other travellers. McCormick uses these accounts as evidence of a vibrant shipping network in the Mediterranean in the eight and ninth centuries. He then backs this inferrence up with "hard" data from recent undersea archaeology, numismatics, and the study of relic hordes.
In the end, McCormick discusses the export of Europeans as slaves to the Caliphate, and, to a limited extenct, Byzantium. McCormick's final argument is that this slave trade was massive, and provided the fuel for the growth of European commerce, growth that was sustained even after the decline of the slave trade.
When all is said and done, McCormick's book is amazing. His arguments and evidence are controversial, and it is easy to predict that this book will be the focal point for scholarly debate for the next generation. Well written, engrossing, and thought provoking, this book is a must for anyone interested in medieval studies or good scholarly debate. The beuatiful maps, charts, and graphs, and the detailed accounts of travellers in the appendices simply add to the value of this book.