Frankly, if I'd been in charge of naming this book I'd have called it something different: 'The Third-Century Crisis', or to borrow from German historiography 'The Age of the Soldier-Emperors'. Or they could have chosen something dramatic such as 'The Collapse of Rome' or 'The Survival of the Roman Empire'. They could even have used one of their own chapter names: 'A World Geared for War.' The current title is so painfully generic that it holds out little hope of being a readable book. Which is why it is somewhat surprising that it manages to achieve a very high level of entertainment while remaining scrupulously accurate.
The book begins (unsurprisingly) with Septimius Severus. All books dealing with this time do so, which is somewhat odd since the empire didn't change so much during his reign as after it. His main contribution was to tie the military more securely to the seat of power, thus limiting the role of senators. His successors had to deal with the increasing instability of the position of the emperor and with the ever increasing frequency of assassination. After the Severans fell, the empire passed back and forth between various generals who raised themselves to the throne only to fall within a few short years. This is what's known as the crisis of the third-century.
Towards the middle of the Third-Century Crisis the book becomes little more than a list of emperors being raised and defeated. This is the same with every book dealing with this time. Nothing can be done about it since these fluctuations are important and yet very little is known about them except for the names and a brief synopsis of their careers. This book does manage a decent summary, helped out by the inclusion of pictures for most of the emperors. There is nothing special here, but there can never be. Southern feels that Gallienus was the biggest innovator before Diocletian, having created the mobile cavalry that would form the comitatenses and promoted the careers of equestrians over those of senators. This section takes up all of the second chapter.
The third chapter deals with the slow recovery from the crisis and the rise of Diocletian. After Diocletian she takes a break to look at the people surrounding the empire, ie: the barbarians and the Persians. Given the changing relationship between Rome and these powers this is an important area to cover. I'm only surprised she leaves it that long. After this comes the concluding chapter which is an analysis of what changed between the beginning of this period and its end.
There's nothing new or shocking about this book. It covers what it aims to cover in an almost entirely chronological way. I'd say that this book works best as an introduction for the beginner. Not that there isn't valuable information in here for more advanced students, but there are other books which analyze events in more detail. Unfortunately most of these are in German. A more in-depth and thorough look at this period is
The Roman Empire at Bay. It is a slightly harder read but makes it easier to figure out where to go for further reading. This book is slightly too polished to make that easy. But don't let that deflect you from reading it. It is a very good book that is accessible to the beginner but not useless to the professional.