I struggled when reading this book. It is excellently researched and very detailed, perhaps too much detailed at times. Contrary to what another reviewer stated, this is NOT a book for "amateurs" or, to use another expression, the "general reader" and the author was specifically NOT targeting this audience. Rather, it is intended as a text book and reference for students, researchers and scholars and in this respect, it is certainly worth a good four stars.
I am no specialist of the Merovingians and the three centuries during which they ruled (AD 450 to 751). I accordingly got confused, at times, and expect that this may happen to many other readers, as "Frankia" was divided generation after generation between 3 to 5 Kings from the same family and all bearing much the same names. This is particularly the case with chapters 9 and 13 that deal with the seventh century.
One of the major weaknesses of this book, when reviewed from a "general reader's" perspective, is that the chapters dealing with the political history of the dynasty assume that the reader already has a firm grasp of the historical outline and of the reigns of the various monarchs. There is no real narrative. Instead, you have various points (and very interesting ones) being discussed and multiple examples drawn from the period under review to illustrate them.
Arguably, I preferred the first and last chapters dealing, respectively, with the Fall of the West and the establishment of Merovingian power, and with the rise of the Pipinids and Charles Martel, but this was largely because I knew more about these periods to begin with. The chapters on ecclesiastic power and its limits, on the place of monasteries, on land and wealth and the culture of the Churchmen are also very good.
The second problem, for which the author cannot be entirely blamed, is that even these chapters are constructed around a limited number of examples which may be too few to be really representative. This illustrates the serious source limitations from which most of the period suffers and which is something that the author also shows rather well.
As a piece of scholarship for scholars, researchers and students, this book may be worth four stars, or maybe even five. However, for a "general reader" - which is the only way I can rate this book - this book, it is not the place to start learning about the Merovingians, despite the quality of its research. From this perspective, I can only give it three stars.
PS: added on 10 August 2012
There are two other elements that I forgot to mention about this book. Both of them are additional reasons for a three star rating.
One is that there is next to nothing on the military aspects of the Merovingian Kingdoms. Given the importance of war - Frankish society was essentially warrior-based, just like other Germanic societies (think of the Angles or the Saxons, for instance) - this is rather surprising, and a major omission, to put it mildly.
The other point is that this book, despite its structure, reads a bit like a collection of chapters that study a specific aspect. Although linked together by the handful of chapters that deal with the political history of the Merovingian Kingdoms, this tends to show. It also reflects what are probably the author's own fields of research. However good his research may be (and it is, at least as far as I can tell), dealing in detail with the Church, monasteries and abbeys without having a word to say about the military organisation of the Kingdoms can only strike you as strange...