The Republic discusses many of the philosophies of Plato, including his theory of forms, the immortality of the soul, the ideal of the philosopher-king, and more. It is written as 10 books, each of which features Socrates engaging in philosophical debate & Socratic dialogue with other characters. The Republic has been commented on by scores of philosophers, writers, and rulers – each of whom seem to respect it for its place in antiquity (at the least), even if they might disagree with Plato on the substance. You can read a few interpretations on this Wikipedia page.
You can purchase the complete works of Plato, the Republic on its own, as well as the graphic novel Action Philosophers at the corresponding links. While not a long work in and of itself, its legacy and impact are nearly unparalleled.