Vegetius: Epitome of Military Science, translated by N.P. Milner, Liverpool University Press, 1993
A classic commentary on the Late Roman army by a contemporary. Although he often harkens back to earlier periods and is full of criticism for the army of 400 AD, Vegetius provides a mass of useful detail, from naval affairs, sieges, logistics, fort duties recruitment procedures and more. A fantastic book. Milner's translation includes lots of up to date scholarly (and useful) notes that make the book readable and even more useful. A workable online translation is available here, but it is slightly dated in places, written as it was by Lieutenant John Clarke in 1767. This may or may not matter to you.
Maurice's Strategikon, translated by George T. Dennis, Univ. of Pennsylvania Press, 1984
An early Byzantine handbook to the legions written by Emperor Maurice (Maurikios) AD 582-602. Although later in date, the army is essentially similar in operation and detail to the Western army of 400 AD, although there is much more cavalry and a new Greek terminology. This book is very easy to read, and full of useful commentary and advice. Organisation, ranks, recruitment, order of march, Latin commands, the tactics of enemy forces, medical care, military law and much more - find out at last, what a foulkon is! Indispensible for anyone who takes their role as Late Roman pedes seriously!
The Later Roman Empire, Ammianus Marcellinus translated by Walter Hamilton, Penguin, 1986
Recognised by all historians as the most reliable and best written account of events in the Late Roman period. Ammianus was an officer within the imperial household troop and followed Julian on his campaigns in Gaul against the Alamanni and then against the Persians. I lke this book alot, Ammianus has a flair for the dramatic, and he paints some pictures of warfare that I will never ever forget.
Of being trapped by the crush of soldiers fleeing into a city and corpses being carried along with them, upright from the press of bodies. Of seeing a corpse leaning against a wall, the head and neck cleaved down the centre so that the two halves rested on the shoulders ... This is a grim war diary of a field officer. There are many political accounts, but Ammianus is a soldier and his concerns are military. If Vegetius and Maurice is the theory, this is the practice. Read it for the account of the Battle of STrasbourg, and for the terrifying retreat from Persia after Julian's untimely death (did I give the end away there?).
The Jewish War, Josephus, translated by G.A. Williamson, Peguin, 1959
Of course this book is out of date! It relates the Jewish rebellion between AD66 and AD70, a full 300 years before our period, but it is a book that will illuminate any re-enactor's vision of the Roman legions at war. Personally, I began reading at chapter 9, chapters 1-8 being a history of the Jewish nation. The narrative kicks off (from chapter 9 ...) with the defeat of a Roman legion by Jewish rebels. That sounds implausible, but it happened. In fact the narrative is strewn with Roman failures and defeats, which is something I love to read about. Quickly we enter the story of the war. It is told by Josephus, a rebel leader who was captured and changed sides. He commanded Jewish forces at Jotapata and the account of that siege is both detailed and gripping. Josephus also accompanied the legions when they marched on Jerusalem. This is a terrifying book that will make you shudder if you are human. The Jews in Jerusalem were fighting a horrid civil war even as the legions approach, and that continued whilst they were being besieged. Read this to discover how ancient warfare was conducted, and what it was like to be besieged. War is hell.
The siege of Jerusalem is a fine example. At times I had to stop reading and say to myself: "this really happened, this is not a novel, this event really took place". Again, its the small things that remain in the memory. Vespasian, scouting the walls with his cavalry, being wounded in the foot by a Jewish slinger. Romans swarming into a besieged city, chasing rebels onto roofs, and one roof collapses with the legionaries' weight, killing many of them. A great, if depressing, read.