Plato's 'Republic' is one of the most important works of ancient Greek philosophy, and one of the foundation pieces of political science and political philosophy of that and subsequent ages. It was one of the first pieces I read when undertaking a political science degree. This translation by Allan Bloom is perhaps the most recent 'Republic' I have read.
Plato was not only a great philosopher, but also a great writer. While few master the classical Greek language sufficient to undertake its study in the original language, the text appears in countless translated forms of varying degrees of integrity. This translation by Bloom is one of the best literal translations - it stays very closely to the original, explaining things that do not translate easily, but avoiding many interpretation issues that often show more of the philosophy and/or politics of the translator than of Plato.
The text is traditionally divided into ten sections, although some scholars see this as being a function of the papyrus and scrolls of original composition more than being integral to the structure of the text itself. One of the interesting features of the Republic is that it was not originally intended for scholars and philosophers primarily, but for the common (albeit educated) reader, and remains one of the more accessible texts of ancient Greek philosophy.
In typical fashion, this is done in a dialogue fashion, with the lead character Socrates (fashioned after Plato's teacher, the great philosopher Socrates, although the words Socrates utters in this and many other Platonic dialogues are undoubtedly Plato's own). There is a discussion on method (the Sophist Thrasymachus shows up early to make disparaging comments about the Socratic method) whilst trying to determine an adequate definition of justice, as well as a discussion on the virtues and/or utility of wealth and old age early in the text. Socrates moves the discussion of justice away from the individual toward the communal, and this is where the political philosophy gets played out in full.
Book II shows the setting out of an ideal city (city-states being the most common form of political organisation in Greece at the time of Plato, with Athens and other cities competing for dominant role). Division of labour becomes an immediate necessity if a city grows beyond a small village setting, according to the theory here. These essentially become classes of people, with different rights and responsibilities, and different expectations of education and material well-being. The guardians or army class is the first one introduced, including an extensive discussion of the type of education and indoctrination such a class should have - this involves political and religious aspects.
It follows from this discussion that censorship is not only tolerated, but selectively preferred. The guardian class is elaborated upon - they are to be divided into rulers and helpers (officer and enlisted class, perhaps?), and they should live separately from the city they guard, owning no private property so as to not be corrupted or corruptible.
After establishing the just foundation of the city, the discussion returns to justice for the individual (interesting to note that what is not discussed is if justice is attainable in a non-ideal city). Justice, after all, is that state when everyone is doing what he or she should be doing, not meddling in other affairs, and exhibiting the virtues of moderation, wisdom, and courage. Justice becomes one of the virtues, and is part of an inner state of the soul of one living in such a society.
Interesting parts of the Republic include the very early idea for equal rights and responsibilities for women, particularly in the guardian class. It is unclear whether Plato was aware of how self-serving his dialogue would seem, since his argument leads to the `natural' conclusion that the only ones who could really be in charge in such an ideal city would be the philosophers. Plato is not an advocate for democracy, and pokes fun quite a bit at democratic structures; he similarly disapproves of most of other types of government (oligarchy, plutocracy, timocracy, etc.) - one can discern the frustrated politician here.
However, the real power of the Republic lies in Plato's remarkable images and metaphoric stories in the second half of the dialogue. These include his expositions on theories of the Forms, and trying to explain what the Good is, and how humankind interprets such things. The images of the ship, the Sun, and the men in the cave are powerful images that have lasted in popular literature since the time of Plato.
This is a classic of Western literature and of world literature.