The first book describes the earlier half of Augustine's life: growing up wicked and committing various sins, cycling through various religious philosophies, coming under the influence of St. Ambrose, who is somehow important, and going through a conversion ritual. At this point, Augustine comes to the important realization that there is no substance to evil, only that it is the lack of God. City of God is basically a 1000 page plus expansion on this point, with some potential meat for early believers in predestination thrown in.
In real life, Augustine spent much of his time rallying his community against invading Vandals. Why? Because he was North African, a fact which is often seized on to demonstrate that the Core Curriculum is not composed solely of dead, white males, as Augustine is an old, dead, and sort of potentially light brownish pied-noir.