At times, The Twelve Caesars is pretty interesting. I've not read that much Roman history, and while this is hardly exhaustive, it does provide a decent look at each figure, as well as provide a good overarching view of the changes in Rome and its politics.
Even so, I frequently found my mind wandering to other topics while reading, and going through most of a page only vaguely aware of what I was reading. Dennison's quite in love with his own vocabulary, which can make his writing a dense at times. I note he uses "grizzly" in situations where I think "grisly" is what he's going for, unless they share the same meaning now. It's only two or three times, but in each case it snapped me out of the sentence and got me thinking about grammar. One more distraction from the book itself, then.
When describing each of the caesars, Dennison largely quotes ancient historians. Plutarch, Dio, Tacitus. His book is itself based on Suetonius' work on the 12 caesars. He says in his introduction he hopes to illustrate telling facets of these different men in these brief overviews, and would consider the book a success if he convinced even one reader to read Suetonius' work. Considering how he describes that fellow's approach to things, I can't imagine why I'd do that.
With Suetonius, everything is portents, signs, oracles, prophecies. There is no significant event which happens where there wasn't some allegedly significant sign of it beforehand. These signs are, naturally, always interpreted in one way, and that is always the correct interpretation given how things turn out. Suetonius might view my life and proclaim my distaste with waffles one morning as a sure sign that I was going to fall and twist my knee later that day, as I just so happened to do. The most interesting thing Dennison revealed to me about Suetonius' work was the idea that character was believed to be unchanging in those days. So if you believed a particular action was the sign of poor character, then that person must always have had that particular failing or weakness. Which puts the historian in the position of having to explain decisions which run contrary to their overall perspective on the person in question.
Tiberius starts out doing fairly well as principate, but gradually withdraws from Rome more and more, grows more prone to having people killed, more imperious, less willing to humor the senate's delusion that it still has any power whatsoever. Whichever Tiberius the ancient historian decides was the true one, they have to jump through hoops to explain the other actions, usually opting for either a malignant outside influence, or that any benevolent actions were a sham (Domitian gets that last one)
The shifting state of the Senate was another interesting part. Augustus pulled a nice little song and dance where he appeared to surrender his position and powers after a certain point, but in reality, he'd surrendered nothing. However, he made the Senate feel as though they were still important, which kept them mostly happy and quiet. Some of the later fellows did this, others didn't, and the latter frequently fared worse. Which is funny, because the Senate rapidly loses any real say in it, as the legions, and especially the Praetorian Guard, become the king-makers. Which it's decided someone is to be removed, they're the ones who do the removing, not the Senate, which mostly sits there doing nothing until the caesar's dead, at which point they engage in a lively round of "I hated that guy so much, I always said so!" But soldiers can be fickle, and it's hard to keep enough of them happy enough that they don't start looking for a new leader in sufficient force to make something happen.
As far as The Twelve Caesars goes, I was more intrigued by the things less directly related to the actual caesars. The perspective of historians - both how they approach it and who is writing the histories* - and how the position was achieved shifting over time. The book's not bad for an overview of the caesars, but I think if that were your interest, you'd be better off seeking out specific books on whichever one you were most interested in.
* There was a note that Tiberius, while not loved in Rome for the distance he kept from it, was quite well-regarded in the provinces, who were used to being handed edicts from afar, and so were less concerned with whether they came from Rome or Capri, than with how smoothly their own lives ran. Perhaps like umpires, the best emperors are those you never notice?