This is Prof. Levick's first imperial biography, the others being on Claudius and Vespasian. All three books are scholarly, very deeply analytical of ancient sources and evidence; they're definitely not meant to be an introduction to Roman imperial history, especially not "Tiberius the Politician".
Not being a historian myself - just a history buff - it would be a bit presumptious of me to criticise Prof. Levick's thesis too much. Basically, she sees the upheavals of Tiberius's life and career - both before and after becoming emperor - as struggles between factions with opposing views on how to approach the government of Rome from a political point of view.
Both factions recognized that the old Republican form of government had become inadequate for the vastness of Rome's armies and provinces, and that a government centred, in the last analysis, in one person - the Emperor, or, more properly at the time, Princeps (First Citizen) was needed.
However, one faction tended to see the role of the emperor as a necessary evil, preferably to be limited to controlling the legions and the provinces where their presence was needed, while leaving the machinery of the Republican constitution still running affairs in Rome, Italy and the peaceful provinces. That is, the Roman/Italian aristocracy would be supreme in Italy and the "unmilitary" provinces, with the Emperor interfering as little as possible. That vision of government, of course, left immense power to the Senate in Rome.
That vision actually corresponded to the theory of the settlement of 27 BC, when Augustus agreed to stop holding the consulship in Rome continuously and shifted the constitutional source of his authority to his provincial commands (and his tribunicial powers), so it wasn't absurt.
Another faction preferred what actually was happening under Augustus, with the Emperor exercising influence, even control, over the theoretical sphere of the Senate in Rome, Italy, and the senatorial provinces. This faction was favored by the non-senatorial plutocracy and the poor and middle classes.
So, Prof. Levick sees Tiberius as a fervorous partisan of the "senatorial" faction, while most of the imperial family of his time - his wife Julia (Augustus's daughter), Augustus's heirs Gaius and Lucius - inclining towards the "popular" faction.
According to her thesis, most of the dynastic events of Augustus's reign can be attributed to the struggle between those factions. For instance, one of the most controversial events in Tiberius's life - his voluntary exile to Rhodes - is explained as a defeated ultimatum to Augustus: stop promoting those boys, Gaius and Lucius, and let's return to a more "senatorial" form of government, or I'll retire to Rhodes.
This analysis is extended to most events in Tiberius's reign, with the "popular" faction represented by Germanicus, Agrippina, and even Sejanus (who never really managed to be seen as the leader of that faction).
I found the above analysis - which is certainly true up to a point, at least - very useful, and it leads to a better understanding of some controversial events. For instance, the debate in the Senate following Augustus's death, when Tiberius was reluctant to assume the sole government of the Empire. This has been explained as sheer hipocrisy (also by ancient authors), or as sincerity. Prof. Levick is extremely convincing when she suggests that Tiberius was actually proposing that the "senatorial" view of the imperial government should be enforced - that is, with an emperor, but also with the Senate assuming greater responsibility over its own spheres - only Tiberius failed to make himself clear enough.
Personally, I felt that this "political factions" explanation was taken a bit too far - surely sheer lust for power and personal resentments also played a role in the dynastic struggles of the Augustus-Tiberius era?
Moreover, while the book covers all aspects of Tiberius's reign, I felt that the political-struggle aspect was over-emphasized in detriment to the economic and military ones.
However, that is a hardly fair criticism, since Prof. Levick's own stated main concern was to analyse the politics behind the often puzzling events in Tiberius's life and reign. And in that she succeeds admirably.
But again, this is not a book for readers without a previous knowledge of early imperial Roman history.