As other reviewers have already mentioned on both the US and the UK site, this is a very good introduction to Roman legionary fortresses. It is also well-structured and covers a lot of ground (three centuries and a half) without seeming superficial. I may have wished to have a bit more on the fourth century fortresses although, to some extent, this is quibbling and a bit unfair because of the plates is a (rather superb) reconstruction of a fourth century fort on the eastern frontier. Brian Delf also does a good job in illustrating a legionary fortress under attack and the main types of buildings to be found within such a camp (headquarters, barracks and bath complex, in particular, with each example taken from a different fortress).
Having mentioned this, I believe this booklet combines four elements which make it outstanding when compared with a number of other volumes in the same collection. One is that there are few, if any, works with such a wide scope that bring together all legionary fortresses. Most of the scholarly literature on the subject tends to focus on individual fortresses, on certain regions or looks at fortresses only as part for a wider discussion of the Roman legions.
A second and very interesting element is the care taken in showing how and when legions got cross-posted from one place to another, sometimes destroyed and sometimes replaced or supplemented by newly raised legions when a major campaign was planned. The author's ability identify where each legion was stationed or moved to another location is underpinned by a huge amount of research since knowledge of which legion occupied which site partly depends upon the archaeological finds at each site.
The third element, and another of the main point that the author makes rather well, is to show the link and evolution from the original marching camps, to the static fortress offensive and defensive bases, mostly built along rivers for supply purposes, to the stone fortresses. Related to this, there also seems to have been a trend towards more and more elaborate defences, with at least some fortresses becoming the nucleus of towns and cities that still exist today (whether Cologne or Strasburg on the Rhine or Vienna or Belgrade on the Danube, although there are a number of other cases as well).
The two sections I preferred and found the most instructive, however, were the "Elements of a legionary fortress and the section on living conditions within such a fortress. Both show that these fortresses developed into real military towns, including workshops and granaries and more generally everything that was necessary to make them as self-supporting as possible. Also fascinating was the degree in sophistication involved in the construction of these permanent bases and fortresses, with baths, but also a complex system of drainage and of water gathering, including by aqueducts in some cases.
Finally, and despite having outlined these common traits, the author is careful to insist on what we still do not know (quite a lot in fact) and to point at what are at best assumptions. He also makes the point that no two fortresses were identical as each and every one was adapted to its environment. If anything, this rather excellent introduction demonstrates, if need be, the Roman talent for engineering. Five stars.