I think a lot of this is going to seem really obvious if you're an American who payed even a little bit of attention in your high school civics class, it's in the federalist papers that you really get the meat of the arguements for the structure and function of the Constitution. I guess I found it hard to get anything really new out of these, but that's probably because things like "checks and balances," " bi-cameral legislature," and "no ex-post facto" are already such well worn pieces of American political vocabulary. It's obviously an important body of writing since it more or less made the case for why the articles of confederation had to be scrapped for something stronger.
And since Hamilton, Jay and Madison were actually trying to convince people, the writing style is very clear and concise. Also, it's a good thing to throw back into the face of stupid demagogues who go around screaming about how no one reads the constitution anymore.