Deeply influential book, "The Social Contract" is a "must read" for anybody interested in the history of political ideas, or even in history. It had a big influence on the French Revolution, and in many movements after it that considered that the individual owes everything to the state.
After reading this book you will be astounded by the insight that Rousseau (1712-1778) showed. He explains us, among other things, the reason for the formation of political society, and the origin of the social contract.
I believe this is a good book to start a study on political ideas. It is simple and well written, it has had an important political impact and can make you curious enough to know more. If you are interested, read also a book about the history of political ideas (for example the one written by George Sabine), because it can guide you to other interesting books, and can give you a deeper insight into the ideas, circumstances and life of Rousseau.