This is an easy-to-read history of the Roman crisis of the third century where the empire struggled with the various changes that were happening in the world. The writing is clear and straightforward and seems to be well sourced. The author makes it clear when sources conflict or leave out information entirely without significantly interrupting the flow of the narrative.
The book is a bit confused at its focus. Most of the book discusses the sequence of emperors and how they came to power and what was happening in the empire to cause such rapid change. Every few chapters covers early Christianity. While there are some intersections between these two strands (emperors did persecute early Christians) there doesn't seem to be a unifying point to these chapters. I feel like the author was fascinated by early Christianity and wanted to discuss it in his book, but a good editor would have culled these chapters quickly. The chapters themselves are fine and well written, but if you are not interested they may be skipped without consequence.