Constitution of the United States

Surprisingly readable, not requiring a lot of background knowledge to make sense. In form, it's just a neutral-tone legal document, but between the lines it tells a story of violent revolution, of tense conflict, and of bitter compromise (see, for example, the "three fifths of all other Persons" rule). Despite these imperfections, knowing what an effect this document had on the world in founding the first Enlightenment state, it's pretty impressive how bold and optimistic these rules were, protecting freedoms and establishing fair processes that are so often the first thing to go when a new regime takes hold in a revolutionary nation. Also in this volume are all the constitutional amendments and the declaration of independence, which tell even more of the story. Well worth a read.