Arguably this is the greatest work of philosophy ever written and it is pretty clearly one of the most important books of all time. It comes the closest to summing up all of Plato`s philosophy in one place.
All of Plato`s works are dialogues with the main character usually being Socrates. The first book of The Republic is a book contains a lively and brilliant debate about the nature of Justice. The next nine books attempt to answer this problem by asking what is the ideal state. Socrates argues that in the ideal state we see Justice written large.
So the Republic addresses fundamental questions of Ethics and Politics, but along the way it also addresses metaphysical questions and epistemological ones.
I have read The Republic many times and a long time ago, I had the pleasure of teaching it on two separate occasions. It is one of those books that you can read constantly for the rest of your life and see new things in it. It is still a book that, while I don`t buy into the overarching plan, I draw on in understanding my life on a regular basis.
Plato was also the most aesthetically appealing writer that philosophy has ever produced. The book is beautifully written and only the Apology and the Symposium, also by Plato, are more beautiful.
For people with no background in philosophy, you will need some sort of guide to the book, or you won`t be able to understand all of it, and what is going on.
Every educated person should read this book. It is the must read of philosophy, being relatively accessible to the general reader. It has also shaped western thought for almost two and a half thousand years. One of the greatest books ever written.