One brief comment about Book XX: in XX.17 Augustine comments on Revelation 21:2-5: 

And I saw the holy city, the new Jerusalem, coming down out of heaven from God, prepared as a bride adorned for her husband. And I heard a loud voice from the throne saying,

“See, the home of God is among mortals.
He will dwell with them;
they will be his peoples,
and God himself will be with them; 
he will wipe every tear from their eyes.
Death will be no more;
mourning and crying and pain will be no more,
for the first things have passed away.” 

And the one who was seated on the throne said, “See, I am making all things new.”  

Augustine makes the provocative point that throughout history, as the City of God has made its way along its pilgrim path, drawing others to join it, it has always been coming down out of heaven. What happens at the end is mere the completion of that ongoing descent.