As has already been mentioned, Plato’s “Republic”, Aristotle’s “Politics”, and Aristotle’s “Nichomachean Ethics”. Plato’s a little easier to read and perhaps easier to stay interested in. Personally I like Aristotle even though he can be rather boring and dry. If you read Aristotle and Plato before Augustine and Aquinas it is pretty helpful.
Much of their philosophy is compatible with Christian philosophy and ethics. For example,
“Temperance and self-indulgence, however, are concerned with the kind of pleasures that the other animals share in, which therefore appear slavish and brutish; these are touch and taste……….self-indulgence would seem to be justly a matter of reproach, because it attaches to us not as men but as animals. To delight in such things, then, and to love them above all others, is brutish” – Aristotle (Nicomachean Ethics, Bk III 1118)
Aristotle pointed out one of the chief problems of democracy long ago,
“Now, to have been educated in the spirit of the constitution is not to perform the actions in which oligarchs or democrats delight, but those by which the existence of an oligarchy or democracy is made possible. Whereas among ourselves the sons of the ruling class in an oligarchy live in luxury, but the sons of the poor are hardened by exercise and toil, and hence they are both more inclined and better able to make revolution. And in democracies of the more extreme type there has risen a false idea of freedom which is contradictory to the true interests of the state. For two principles are characteristic of democracy, the government of the majority and freedom. Men think that what is just is equal; and that equality is the supremacy of popular will; and that freedom means the doing what a man likes. In such democracies everyone lives as he pleases, or in the words of Euripedes, “according to his fancy”. But this is all wrong; men should not think it slavery to live according to the rule of the constitution; for it is their salvation.” - Aristotle (Politics, book 5 1310a9)
Here’s a great quote from Plato’s Republic that I think is pretty relevant today,
“Whenever great numbers are gathered to sit in the assembly, in courts, in the theater, in camp, or in any of the mob’s communal gatherings where with stupendous uproar they cheer and jeer actions and speeches, both to excess, shouting and applauding till even the echoing rocks and the place where they’re sitting join in to redouble the racket of their cheering and jeering. How do you think a young man’s heart will react to that? What private education could hold up and not be washed down the river of cheering and jeering, carried wherever the current may take it, so that he calls what they call beautiful and ugly, pursues what they do, and becomes like them?”-Plato (Republic, book 6 492b-d)