The Social Contract by Jean-Jacques Rousseau. I found the writing dry, dry, dry, and so very abstract that I frequently had to try to imagine what real-world situation was being described. There are many passages like this:
It follows from this dual relationship that the geometric progression between sovereign, prince and people is by no means an arbitrary idea, but a necessary consequence of the nature of the body politic. It follows further that one of the terms, namely the people as subject, is represented by unity, every time the square of the ratio is increased or diminished, the simple ratio increases or diminishes in the same way, and the middle term, the government, is in consequence changed.
That said, I am glad to have read it. Rousseau argues against any natural rights to power, and for the concept of a "social contract." The social contract is the free agreement of all the citizens of a state to cooperate in securing everyone's rights and interests. He lays out the components of governments that truly represent everyone's interests. He discusses potential pitfalls, and examines the successes and flaws of the Roman style of government. It's a foundational text, and it's one of the sources of modern ideas of representative government.
And it's not completely devoid of linguistic pleasure. The opening is wonderful:
Man was born free, and he is everywhere in chains.
Some other memorable lines:
If there were a nation of Gods, it would govern itself. A government so perfect is not suited to men.
(Referring to hereditary monarchy): When someone is brought up to command others, everything conspires to rob him of justice and reason.
Where rights and freedom are everything, inconveniences are nothing.
Mixing It Up