I am not an historian but have a rather eclectic reading list. I read this book recently and was impressed by its scholarship and even more by the story it tells... actually, its more about the story it debunks. Wynn gives us first) not an historical version of the "just war" theory but rather a detailed picture of Augustine's writings related to war and military service since it is Augustine to whom the theory is attributed, and second) the actual context of where such "just war" roots really came from and [SPOILER ALERT] it wasn't Augustine but rather different factions of the Roman empire trying to justify their civil wars to the people. So while Wynn is either too nice or too politic to put it so bluntly, the entire "just war" theory, if it really exists in terms of morally justifying a war, is a modern construct in search of some made up history attached to respected theologians in order to make it seem part of our inherited culture, and if that theologian can be part of the Christian history, so much the better.
With respect to the prose, I found Wynn's style easy to read and was often entertained by subtle but pointed witticisms. If you have an interest in history and especially Christianity and the Roman empire, I think you'll enjoy this book.