A biography of Julian it is - but a description of the death of the pagan world it is not. There is only one mention of Theodosius I can remember and he was responsible for decapitating paganism and who knows, opening the doors to Islam as well - when the Romans gave wings to monotheism.
This is a very easily readable sketch about Julian's life - not an intimate portrait. Starting at Julian's heroic death, it takes us back to his unlikely rise through the jaws of Emperor Constantine's children and their filial, power struggles as emperors of Rome - including the Eastern Empire which Julian was to help the Romans to lose. The book is richly sourced. The end chapters describe Julian's impact and reputation - I'm not sure I agree with the author's appraisal of good vs bad. Julian stands out as a human being, a failed hero, a shadow of Alexander the Great.
Julian was not really an Apostate as he was labelled given that Paganism was still alive at his time - even if it was a receding force. It was in the fifth century that Paganism really died and the civilisations of Egypt and the near East in a pagan context were obiliterated.
I get the impression that Julian tried to impose a form of Paganism that was not necessarily true to tradition as a "standard religion" (like Christianity) - and paganism never worked like that. The gods were not with Julian when he went for Sharpur the Emperor of Persia after which he met his death.
We get a good impression of the later Roman empire and one of the few emperors you can really admire. For his laws, his intellect and strangely - his tolerance as well. I believe the book should lead to further reading.
I don't think this book does any justice to its subtitle - (and the death of the ancient world) you would have to go elsewhere to find this - to the 420s CE.
Overall, a very digestible, well explained compendium with excellent references.