Here are a few great lines from article 28 of the 1530 Augsburg Confession.
“…the Church’s authority and the State’s authority must not be confused. The Church’s authority has its own commission to teach the Gospel and to administer the Sacraments [Matthew 28:19-20]. Let it not break into the office of another. Let it not transfer the kingdoms of this world to itself. Let it not abolish the laws of civil rulers. Let it not abolish lawful obedience. Let it not interfere with judgments about civil ordinances or contracts. Let it not dictate laws to civil authorities about the form of society. As Christ says, ‘My kingdom is not of this world’ (John 18:36). Also, ‘Who made Me a judge or arbitrator over you?’ (Luke 12:14). Paul also says ‘Our citizenship is in heaven’ (Philippians 3:20). And, ‘The weapons of our warfare are not of the flesh but have divine power to destroy strongholds.’ (2 Corinthians 10:4).”
I see that the Westminster (CA) bookstore carries the paperback edition: Concordia: The Lutheran Confessions for around $17. There is also a hardcover edition – check it out at CPH for around $32. I haven’t compared the two, but I think there’s a slight difference (though the text should be the same). Either way, this is a great resource.