Michael Simkins 'The Roman Army from Hadrian to Constantine' was first published in the 1970's, which makes it a bit outdated, considering the numerous strides made in historical scholarship and archaeology since the book's publication.
As a result, much of the development of arms and armour during the third century 'crisis' seems to be absent from this book, as are the developments taking place in the Roman militray during the reigns of Diocletian and Constantine.
The contents of the book are also puzzling. Following a 4 page chronology of the history of the Empire during this period, the author then sets out on a tangent about Hadrian's Wall for about 12 pages. He discusses the building of the milecastles, the medical and funeral services and even the religious cults of the soldiers stationed there. I am not sure what this has to do with the late Roman army, and it seems as if the author was trying to use this section as filler, considering that there was not much information on the military of this period.
The remainder of the book seems outdated as well. The author discusses the arms and armour of the period, yet all he writes about is the gladius, lorica segmentata, the imperial gallic helmet and the rectangular scutum - items that were beginning to fall out of fashion by the mid third century, and had completely disappeared from the Roman armoury by the age of Constantine.
His descriptions of the Roman arms and armour from the high Imperial period seems even stranger considering that a few of Ron Embleton's illustrations show Roman soldiers wearing a 'Deurne' helmet and another with an 'intercisa' helmet - both correct for the late Roman period. Yet these aren't propelry acknowledged in the text, except for one or two sentences on the last page of the book.
Still, I can recommend the late Ron Embleton's colour plates which are detailed and expressive.
For those interested on the army of the late second to fourth centuries, it is probably best to look out for other titles - although, that said, this book would do as good introduction to the Roman military.