"Emperor of Rome," by Mary Beard.

Ancient Rome was a republic, until it wasn't. That's where Emperor of Rome begins.

The author, classicist Mary Beard, examined the history of the republic in her previous book, SPQR: A History of Ancient Rome, 2015. It's not necessary to have read that one ( I haven't yet) in order to enjoy this detailed, and very entertaining 2023 examination of the Rome under the Emperors. It includes the usual suspects, (Caligula, Commodus, Marcus Aurelius, etc.), some crazy-cruel, and some thoughtful administrators.

Of special interest in this time of divisiveness and threats to US democracy are two ways that Julius Caesar's appointed successor, Octavian, known as Caesar Augustus, took control and consolidated power:

1. He nationalized the military, and became the supreme commander of all Roman troops. No longer were "Big Men" (rich and ambitious) able to raise and finance troops which were loyal to them and not the ruler of the country. No longer could military commander lead a "Triumph" parade through Rome as they did during the Republic--that privilege belonged only to the Emperor, as the head of Roman military might.

2. He ended real elections. Some elections were still held, but in a form that siphoned actual power to the emperor.

A major flaw in Rome's transition from republic to empire was the lack of a clearly defined way to transition from one emperor to the next. Hereditary primogeniture, where power flowed to the firstborn legitimate male child, didn't always apply in Roman tradition. The emperor might adopt an adult male of his choice to succeed him, but the violent ways many emperors ended and were replaced shows how important a "peaceful transfer of power" is to the stability of a nation.

Not just a dry history--lots of dishy gossip and reports of truly bad behavior, beginning with Julius Caesar, (assassinated 44 BCE) and ending with Alexander Severus (assassinated 235 CE).