This book is very timely for Christians today and, unfortunately, in the days to come. Viret lived through times that Christians in America have never really experienced in our country's entire history - direct and concentrated persecution coming from the civil magistrates, as well as armed rebellions specifically centered around religious issues.
One thing that really stood out to me is how he talked about how if Christians have a civil magistrate who is persecuting them for their faith, and there is no God ordained lesser magistrate to lead resistance to him, then they should view that as part of God's sovereign plan and accept the persecution, knowing that God could provide the leader if He so desired. Now I think I may argue for a definition of lesser magistrate that's a bit broader than his (and that is something that I wish had been covered in more detail), but none the less the situation that he describes still could certainly happen, particularly where Christians don't make up even a sizable minority.
Viret was writing in a very different cultural context than modern conservative America. He doesn't spend time talking about God given rights that men posses. Although I think that there may be something to those doctrines that he may have been missing, that might make him all the more valuable to point us back to scripture, rather than our socially or politically learned principles.
This book is a collection of sections from several of his writings, and as such it is not entirely cohesive. I'd suggest you think of this as food for thought rather than a comprehensive approach to the subject.