This is a relatively good summary although it was always going to be a rather daunting and even an impossible challenge to cover a thousand years of Byzantine military history within some 230 pages. This is what this book attempts to do and, unsurprisingly, the results are somewhat mixed. Some periods are covered much better than others, while the last two and a half centuries are hardly covered. The exceptions are a couple a couple of remarks drawn from Mark Bartusis' book on the late byzantine army after 1204, including one alluding to the limited size of the forces that the Byzantines could gather by that time - no more than 5000 according to this author, although this seems to be a plausible "guess-estimate" more than anything else. The problem with this book is that Michael Decker has no room to even discuss this number.
Essentially, this book is a summary that brings together in a single volume the main elements and findings drawn from several other works, including Haldon's "Byzantine Wars" and his Warfare, State and Society" but also his book on "Byzantium in the Seventh Century" or Mark Whittow's "The Making of Orthodox Byzantium", to name just these. There is therefore little that is original but it is mostly well presented and the book manages to be a worthwhile introduction for a general reader wanting to get acquainted with Byzantium, its army, and its endless fight for survival.
There are however a few additional glitches. One is that the book could have done with some proof-reading or perhaps a better editor. The Emperor Romanus Lecapenus' reign did NOT, for instance, end in 948 and there are a number of other such glitches scattered across the book. Another, which is the consequence of the book's size, is that some aspects are covered in what can only be termed as superficially. Perhaps the most blatant example of this is the first chapter presented a "Historical Overview" - a summary of a thousand years of history- in a mere thirty pages including almost ten pages of maps (which are rather good, by the way).
A third shortcoming is the last chapter, which bears the same title as the book and is supposed to bring together the aspects covered by the author in all previous chapters to explain what constituted this "art of war". I simply was not convinced by this chapter. I did not find that some of the points made by the author, such as the adaptability of their forces and the use of combined arms, were quite unique to the Byzantines. Moreover, the inclusion of developments on Greek fire in this last chapter looked like a bit of an afterthought.
Having mentioned these limits, the book also has a number of strongpoints, particularly for the so-called "general reader". In addition to those above and keeping in mind the limitations already mentioned, the six core chapters, each of which deals analytically with one aspect of Byzantine warfare (strategy and tactics; leadership, recruitment, organization and training,; equipment and logistics, and so on), are mostly clear and to the point. The chapter on Byzantium's enemies is probably the weakest: it is largely a paraphrase of Maurice's Strategikon and, with only twelve pages, would have deserved to be considerably more developed and detailed.
The chapter of Byzantium at war is the one which resembles Haldon's "Byzantine Wars" the most, but with some significant differences as well. It includes context for each of the campaigns and battles and the author had been careful to limit overlap with battles already covered by Haldon or Corey in his Road to Mantzikert. So you will find a description of the Vandal War of Justinian, but not the Persian or Italian campaigns, the campaigns of Nikephoros II Phokas and Basile II's battle of Kkeidion, but not other battles against either the Arabs or the Bulgars. The battle of Semlin (and not "the battle of Sirmium", as one of the maps page 203 mentions) is also covered by Haldon. I was a bit surprised to not find a description of either Mantzikert or Myriokephalon, although this might be explained by the fact that Michael Decker has only selected battles where the Byzantines were victorious. Other good points are the section on siege warfare and on the campaigns of John II Komnenos (largely drawn from John Birkenmeier's "The Development of the Komnenian Army"), who is, to some extent, largely unknown and whose reign and military consolidation allowed his successor Manuel to conduct a widespread, brilliant but ultimately ineffective policy across the whole of the Mediterranean.
I also had an additional problem with the book's contents, when compared to the book's title: the book is essentially about Byzantium's army and has very little to say to say about its navy except when dealing with Greek fire. Byzantium's navy is therefore another aspect of "The Byzantine Art of War" which has been somewhat neglected by the author. Also, another chapter could easily have been added about the "byzantine art of war" of winning conflicts without having to fight. While there are a number of references and descriptions of some of these aspects, for instance in the section on strategy and tactics, the importance of Byzantine diplomacy (including bribes, gifts and buying the peace, subversion, assassinations, use of religion for political advantage, hostages, alliances with the enemies of their enemies) has been somewhat underestimated. This was also very much a key component of Byzantium's "art of war", if ever there was one, and it would certainly have deserved to be better treated.
Three and a half stars for a book that would have deserved to have another hundred pages or more in order to fully cover its rather over-ambitious (but fascinating) subject.