This is one of the better Osprey titles. Although it is probably not the "essential reading" that a somewhat over-enthusiastic reviewer has portrayed it to be, this is a book written by an author who clearly knows his topic sufficiently well to be able to summarize it without leaving any of the important pieces out. The author - Ross Cowan - finished his PhD thesis in 2002 precisely on this period (the title was "Aspects of the Severan Field Army") and published this Osprey title the year after. In particular, Ross Cowan's research focused on The Praetorians and the Legio II Parthica, the favourite and elite legion under Septimius Severus and his immediate successors.
The other two main qualities of this book are to focus on a period which had traditionally been rather poorly covered: the third century crisis and, perhaps more accurately for this title, to show how the Roman Legionary and Roman Legions evolved and responded to this crisis. Two points were of particular interest to me. One was to show that the need for a central reserve force emerged already under the reign of Septimius Severus, and was further developed by Gallienus (but with a stronger emphasis on cavalry), and by some of his immediate successors (Aurelian and Probus) well before the Tetrarchy and the reign of Constantine. The second was to show that, contrary to what sometimes used to be asserted, this did not result in a demise of the Legionary but simply in a breaking up of the Legions into smaller components. Some of these would remain stationed in one or several forts on the frontiers whereas other components, which had probably started off as ad hoc detachments (vexillatio) for specific campaigns became the "new" Diocletianic legions of 800 to 1200 strong.
The structure of this book is the traditional one (terms of service, changes in command and in army organisation, legionary equipment), but the author's grasp of the subject and the rather more limited timeframe makes it more valuable and allows for more information to be delivered. The section on the lanciarii was also very interesting. The piece on the siege of Doura Europos, and the Roman Army units that defended it against the Persians makes also for interesting reading for anyone that has read Sidebottom's Fire in the East. I found it a pity that we could not be treated to the full siege but I appreciate that the author had space limitations. Finally, the demise of Legio II Parthica under Diocletian is also interesting, and somewhat unsurprising given their association with the Severans.
I also very much liked the plates, which is also unsurprising since they were from the Angus McBride. The battle scene between Legio II Parthica and some of the Praetorians at Immae AD 218 was, however, a bit problematic for me. Contrary to another reviewer, I had no problem with the living eagle used as the legion's standard, although I felt perhaps a bit sorry for the poor bird which would have been somewhat stressed and powerless in the middle of the bloody struggle. I was, however, somewhat surprised to see both elite forces fighting it out in their tunics, without armour or helmets. While I do not doubt that this actually happened, as described by the author, I would have appreciated if he had explained this rather unsual episode to us in a bit more detail.