The Wealth of Nations by Adam Smith. It was incredible.
The scope of his brilliance, I mean its fucking profound the things he said. Its also pretty amusing he wrote about it during the American Revolution while working under Townshend (of the Townshend Acts fame) for the British government, so his insights on "the troubles in our American Colonies" have historic significance. The book was published in 1776, the same year as The Decline and Fall of the Roman Empire, and I don't know what it is about books like this, but they're timeless. People will be reading both books hundreds of years from now. I don't even think thats what Smith wanted, because he very casually references his contemporary philosophers, without knowing for certain that history would remember "Mr. Hume."
I totally recommend it. Its long, but not a difficult read. The language is straightforward.