For those with a serious interest in the Roman army and more widely in the impact of the Roman empire on provincial populations, I have no hesitation in recommending a book that came out late last year - Ian Haynes, The Blood of the Provinces. The Roman Auxilia and the Making of Provincial Society from Augustus to the Severans published by the OUP. As the title says, its focus is on the non citizen soldiers of the early imperial army, primarily the auxiliaries of the cavalry alae and infantry cohorts. It is not simply a study of the history and organisation of these units, but more about what the maintenance of perhaps two hundred thousand non citizen soldiers actually meant. Along the way, it examines the experience of military service, looking at everything from language to the environment, routine, ritual and religion of army bases and the wider military community.
Ian and I are very old friends, and started out doctoral research on the same day a couple of decades ago. At the time, we were the only two graduate students at Oxford working on the Roman army of the principate, and it was a great asset to have someone else with whom to bounce around ideas. Ian's book began as his D.Phil. thesis, but over the subsequent twenty-odd years has developed and grown into something far broader. It has been a great pleasure to discuss many of the ideas many times. Graduate students readily feel certain about things, but as your knowledge increases everyone tends to get less confident. Ian's ideas have been shaped by extensive reading and in particular a wide experience of archaeological excavation and investigation in a number of countries. This is an academic book, and so the more knowledge a reader has then the more he or she will be able to engage with the ideas and profit from the book. I won't try to summarise the arguments, because they cover so many different themes. Not all the questions can be answered, but simply asking them and pushing the evidence as far as it will go is highly stimulating. All in all not the first book to read on the Roman army, but for those with a fair existing knowledge I think it will prove to be one of the most rewarding.