On both the UK and the US sites, there seems to be, at least this time, a bit of a consensus regarding how good this book is. As far as I am concerned, determining whether it is slightly better or slightly less good than the two previous volumes is a rather moot point since I rated all three of them as five star books. What may be of more interest for potential readers are the reasons for attributing the highest possible rating to this book.
There are essentially three of them: excellent plot, excellent historical research and superb characterization. This does not imply that the book is "perfect", even assuming that this was possible. However, and despite taking a very close look at it, and even double checking a number of points - this is at least my excuse for not posting this review earlier! - I could only find a few quibbles barely worth mentioning, as opposed to any significant drawback that may have justified a lower rating than five star rating.
First of all, and as others have mentioned, the plot is fast paced and manages to be rather original, despite being the continuation of the previous volumes and despite the rather large number of "swords and sandals" novels set during various periods of Roman history that are already out there. This is no mean feat in itself, especially when you consider that the period covered in this volume - the last years of the reign of Emperor Tiberius and the first half of the reign of Caligula - did not see any major campaigns or battles taking place. You will nevertheless get plenty of violence and blood-letting in this book which is all about power plots, intrigues, murders and terror in Rome, and ruthless riots and policing in some of its unruly and/or strategic provinces, in particular in Cyrenaica, Judea and Egypt. You might end up disliking the book's plot, or even finding that this or that piece of it and the involvement of the Flavian brothers are somewhat far-fetched, but I would be very surprised if you find this book boring.
Then there is the historical context, the author's research and the ways in which the author has chosen to interpret and present it. Here again, I found it very difficult to take exception with any of these items (and believe me, I certainly tried!).
The author has taking a number of liberties and come up with a number of interpretations, but then that is to be expected in a historical novel. When this has been done, however, he has taken great care to mention it explicitly and, as far as I can tell, in a rather systematic way, in his author's note. This is not something that all writers of historical fiction take the trouble to do and, as far as I am concerned, this is also one of the book's qualities. I believe he has also mostly managed to get away with it, again, at least as far as I am concerned, for at least three reasons.
The first has to do with the contents of the written historical sources. We know rather little about Vespasian's early years (and the same applies to his brother), apart from a few scattered anecdotes which the author is careful to use. This gives Robert Fabbri a lot of license when building up his intrigue and putting the Flavian brothers right in the middle of it all.
Second, the Roman written sources (Tacitus, Suetonius and Cassius Dio) all wrote well after the reigns of Tiberius and Caligula, and even after the reign of Vespasian himself. They were also quite biased and quite eager to show up the Julio-Claudians (Tiberius, Caligula and Nero, in particular, but also Claudius to a lesser extent) in a rather unfavourable light, to put it mildly. This was particularly the case since the first two were, if I remember correctly, writing during the reign of Trajan (Tacitus) or Hadrian (Suetonius) and the third wrote under the Severans. In all cases, the "new" Emperors of the time - and the authors writing during their reigns - were more than eager to distance themselves from the previous dynasty and to portray the members of this dynasty as unworthy in general and either half-wit cripples (Claudius in particular) or mad tyrants and monsters (Tiberius, Caligula and Nero). The sole exception was Augustus (Octavius), the founder of the Empire, which they all adopted as their role model.
What you therefore end up with is a rather extreme divide between "good" emperors and "bad" ones, with the, respectively, positive or negative features being probably somewhat exaggerated (the "good" ones are excellent, the "bad" are simply awful monsters capable of just about any meaningless crime or atrocity). These kinds of bias, while making life extremely difficult for a historian, are wonderful for an author writing a historical novel as he can have a field day in developing some truly hideous characters - real monsters - while being fully supported by the sources. This is exactly what Fabbri has done in this book with both Tiberius who, at least initially, may not have been entirely awful and was certainly a competent emperor, and, of course, Caligula, the archetype of the mad paranoid and entirely unpredictable emperor. In both cases, but perhaps even more for Caligula than for Tiberius, the paranoid and murderous atmosphere at the Imperial Palace that they had to live in since their youth goes a long way towards explaining their atrocious behaviour once they became emperors. This is also something that is very well shown, especially for Caligula, in both this volume and in the previous one.
The third reason is the way that Fabbri manages to convey the rather atrocious climate of terror, abuse, tortures and arbitrary executions and casual murders and rapes that senators and their families had to live through during the reign of Caligula. It is so vivid and oppressive, and entirely based on the historical sources that it "sounds and feel" true, regardless of whether certain specific features and points of detail have been exaggerated or not.
As very well shown in the book, the sheer uncertainty regarding their fate obliged all of the Senate to transform themselves into the most abject sycophants in what were rather abject attempts to ensure their own personnel survivals. The same - or something very similar - also happened under Nero. What may have made it somewhat worse under the reign of Caligula was that he seems to have been so unpredictable: no one seems to have been able to know to what extent he was mad or to what extent his terror tactics were deliberate and vengeful. He also took an obvious sadistic pleasure in tormenting the Senators and their families (including his uncle Claudius) both morally and physically. All these traits are also superbly presented and described in this book.
Finally, another set on gems with regards to the historical context are the author's descriptions of Rome and Alexandria, in particular, with their various monuments (the Alexandrian Pharos, one of the seven marvels of the world, was a particularly good piece for instance). To a lesser degree perhaps, he also manages to achieve something similar with both Jerusalem and Cyrenaica (Appolonia and the Siwas oasis and temples). Here again, the riotous atmospheres in both Alaxandria and Jerusalem, and the fact that both are described as powder kegs just waiting to explode, is also very well conveyed (and backed by historical evidence).
As I have already hinted at when mentioning the way Fabbri presents and describes the "bad" emperors, the third main and very successful feature of this book is the extreme care that the author has taken in characterization.
This starts with the main character (Vespasian) that the author has been careful enough NOT to present as a flawless "hero" but rather as a gifted young up and coming senator, with all his flaws and limitations. He is not above quite a bit of bribery, nor is his brother Sabinus. His personal life becomes somewhat complicated, to put it nicely. He is ready to abuse his position if this can get him into bed with a pretty face. Both brothers are typical senators of their time, and so is their uncle. All went, as mentioned above, rather far in order to survive and save their skins, and our "heroes" were no exceptions. This interesting feature contributes into making the characters more "realistic" and perhaps also more human if not also more sympathetic.
The other main characters of the book are also very much based on and derived from the written sources, starting with the scheming Antonia, daughter of Marc-Antony, and the real power behind the throne in Rome during the last seven years of Tiberius' reign, once Sejanus had been removed. Even Herod Agrippa and Alexander the Alabaster are very good. My only little gripe, if I really need one, was Paulus which I found to be a bit too much of an "arch-villain", a bit of a caricature and not as convincing as the other characters.
While all this going for it, and to cut an overlong story short, I simply could not see any way to rate this book less than five stars...