To me Justinian was the last Roman emperor in the classical sense; off course there was still a thousand years of Roman emperors to go that would rule from Constantinople and Nicea but what makes Justinian the last Roman emperor to me is that he ruled and acted in his ruling as if the whole of the former roman empire should listen and he made a lot of people do so when they hesitated.
However that would limit my definition to the military conquest and that, as Peter Sarris emphasizes is falling into a double trap; namely over emphasizing the military and underappreciating the long term impact of Justinians impact on laws and religion as a tool of the state; hence the title, an emperor who rules, a soldier and a saint for his involvement in religious debate and theology. By reforming the rule of law and state affairs, he both consolidated what would be Roman law as well as usher in its end in its classical understanding of it, however the book is not about him alone.
Peter Sarris takes a serious effort to underline the court of Justinian and the team effort that his rule was, empress Theodora, the taxman John the Cappadocian, the general Belisarius, Tribonianus the jurist and the historian/ eyewitness Procopius of whose writing a lot of our knowledge and image of the emperor is based. This is not a book either glorifying the great man emperor but nor is is full on embracing the demon king and his devilish harlot that Procopius made of him in the secret history, the last unfinished manuscript of his histories. For the question, to Peter Sarris, is far fom clear cut; was Justinian the cause of the fall of the eastern Roman empire? Was his attempt at restoring the whole of the empire the reason it all fell nearly apart 60 years after his death? Besides that big question a lot of detail is given where possible to the impact of Justinians wars, legal reforms and religious policies on as wide of a mass of the population as possible. In particular the chapter on the elite family in Egypt as well as a town in the Fayoum region are particularly enlightening.
In the end it is as Sarris indicates quite impossible to deny the impact his reign and court had on the western/ Mediterranean world and any one of the big events in his reign, the Hagia Sophia, the plague, the Nika riots to name a few would have been The big event of many a ruler but for Justinian's they are but chapters in his life. However I am far less forgiving of the man then Sarris is; even if the armies where small and the energy given defensible in the wider scheme of things, I still do think Justinian caused the end of his own state's fortune in the decades not just after his death but even during it when the Avars came knocking on the door. With every new war in the west; Africa, sicily, Italy, Spain Justinian added another file on the pile for the state to handle and as his rule became so court focused it meant all of it was fighting to get his attention. His opportunistic jumping at chances for success made him leave things unfinished and this is not just something we can say so easily so many years removed even at the time people were concerned about the state of affairs. I would never deny his long lasting impact but by trying to be the emperor of ROME again, Justinian ensured his successors could only be the rulers of Byzantium. I'dd argue for that point anytime