Chapters 3-5: (with a dash of 1 and 2 thrown in)
The central theme here seems to be that the gods protect no one. How silly and empty then is the claim that Rome has been sacked because of unfaithfulness to the gods! In fact, what we see when we look at the relationship between the gods and the city that worships them is that it is the city who protects the gods, not the other way around. "Would it not be wiser to believe, not that Rome would never have fallen into so great a calamity had not they first perished, but rather that they would have perished long since had not Rome preserved them as long as she could?"
More specifically, Augustine reminds us of the worthlessness of idols, those gods which "with trembling hands/ He bears... The gods of her domestic shrines/ your country to your care consigns." These statues offer no safety to those who bow before them.
I was at a large-ish Wednesday night Bible study once (200+ people) going over a verse in the New Testament condemning idolatry. While I've forgotten which verse exactly it was, I have not forgotten that the study leader asked who in the room had been converted out of idol-worship. More than 20 people raised their hands, which I found surprising since in the modern world we tend to think of idols as those things ignorant savages bowed down to in the murky past. The study leader then went around the room and asked everyone very briefly to give an overview of how their view of idols had changed. Many of the people from non-Western nations talked about rejecting family worship and on occasion even throwing out family heirlooms. Those from Western nations (especially America) tended to talk about cheap trinkets and statues given by older relatives on important occasions. But all of them talked about the change from viewing the statues as reverential objects of religious power which offered protection and safety from a dangerous world to aspects of their former enmity with God that had been resolved on the cross.
The point of this seemingly-tangential story is that I think our immediate temptation on reading a passage like this in Augustine (or in the Bible, for that matter) is to allegorize it. Whether talking about idols specifically or pagan gods in general, we immediately want to jump to the question of "what are the modern 'idols/gods' that we worship?" And then talk about food or television or jobs or family or whatever. And while I think that is a legitimate application, I also think we've got to remember it's only ever a secondary application. The primary application is that idolatry and worship of the pagan gods are sins and to be repented of and rejected. Only once we've cleared that hurdle can we move on to the broader interpretation. (And in case you're wondering, yes, there are still pagans around even in America--to say nothing of the broad number of religions that still worship idols.)