The republic by plato
If you want to really understand the Republic, this is a great source. The professor does a great job of relating the info to a modern-day audience. It's not just the book but also the application which draws great analogies to modern understanding. This book is a great class on the Republic. They also discuss the theory of forms, the immortality of the soul, and the role of the philosopher and of poetry in society. In the dialogue, Socrates talks with various Athenians and foreigners about the meaning of justice and whether the just man is happier than the unjust man.
It is Plato's best-known work and has proven to be one of the world's most influential works of philosophy and political theory, both intellectually and historically. The Republic is a Socratic dialogue, authored by Plato around 375 BC, concerning justice, the order, and character of the just city-state, and the just man.