The Prince by Niccolò Machiavelli is a manual for rules who want to exercise power and authority aggressively. And it doesn't waste time on insights into virtue and justice.
Machiavelli says effective leadership has nothing to do with being a good person (it's about being effective). And he wrote this book for Lorenzo de' Medici (who probably never read it) after getting fired and tortured, so there's this "screw it, this is how power actually works" vibe to the whole thing. Rulers should look virtuous while actually being ruthlessly pragmatic. It's better to be feared than loved. But one should ideally avoid being hated since that tends to end with an assassination. You can read more if you click the title.
My Notes
Machiavelli writes that behavior in public and the actual actions of one should be different. And anyone exercising authority in the former way will get crushed and taken over. He introduces a term called "virtù" (not virtue, interestingly). It is the ability to adapt and do whatever the situation demands, morality be damned. "The ends justify the means" probably came from this idea. And he also talked about how it is harder for the new rulers than the hereditary ones. He says it is because people hate change. So, new rulers have to get rid of threats quickly, effectively, and completely. Half-measures could cause rebel groups to form.
The book's packed with tips that would get anyone canceled today: commit all your evil at once so people forget faster, but linger the benefits slowly so they're remembered for longer. And there are a lot of them: Use religion cynically to control people; Break promises when keeping them hurts you; Don't appear weak or indecisive; Mercenaries are expensive cowards.
He also pulls a lot from historical examples. Specifically, the Italian city-states. Machiavelli praises Cesare Borgia. He writes that Borgia was effectively brutal (though he ultimately failed, which Machiavelli blames on bad luck). Francesco Sforza conquered Milan and kept it. Machiavelli calls it success. But Sforza's son fails the state after inheritance. This was Machiavelli's way of proving that founding your own power is better than inheriting it.
The book became famous soon after. But not for good reasons. Machiavellian became synonymous with manipulative scheming. So the Catholic Church banned the book promptly, which probably helped sales. Critics called him immoral, cynical, and even satanic. Defenders argue that he's just describing reality plainly rather than sugarcoating comforting fantasies about philosopher-kings.
We don't know if Machiavelli was evil or just giving some legitimate political advice. But the fact that this book is still read even after 500 years means it was and still is influential.