What did Augustine say about the love of God, better known as Christian charity? Is it a self-interested love, one that seeks God as a means to happiness? Some have interpreted him in this way. The following are some key texts in Augustine that have been (wrongfully I think) interpreted this way. I have included numbered sources, commentary, and some quotations.
1. Augustine, “The Spirit and the Letter,” in Augustine: Later Works, edited and translated by John Burnaby, The Library of Christian Classics (Louisville, KY: Westminster John Knox Press, 2006), 36 , 51 [235-236]. Augustine considers the gift of grace primarily in terms of love, and love as caused by faith. This is also how he interprets Paul’s doctrine of justification—God’s making us love God by the gift of faith, which makes us likewise delight in doing whatever he commands.
2. Augustine, On Christian Teaching, translated and edited by R.P.H. Green, Oxford World’s Classics (New York, NY: Oxford University Press, 1999), I.35-37 [25-26].
Augustine admits that even in someone’s compassion for the needy
… somehow there also results an advantage to us, since God does not let the compassion we show to the needy go unrewarded. This reward is the supreme reward—that we may thoroughly enjoy Him and that all of us who enjoy Him may enjoy one another in Him. For if we enjoy one another in ourselves, we remain as it were on the road and put our hopes of happiness on a human being … When you enjoy a human being in God, you are enjoying God rather than that human being. For you enjoy the one by whom you are made happy, and you will one day rejoice that you have attained the one in whom you now set your hope of attaining him. … Yet the idea of enjoying someone or something is very close to that of using someone or something together with love. For when the object of love is present, it inevitably brings with it pleasure as well. If you go beyond this pleasure and relate it to your permanent goal, you are using it, and are said to enjoy it not in the literal sense but in a transferred sense. But if you hold fast and go no further, making it the goal of your joy, then you should be described as enjoying it in the true and literal sense of the word. This is to be done only in the case of the Trinity, the supreme and unchangeable good. Augustine, On Christian Teaching, I.35-37 [25-26]
Augustine makes a distinction between loving someone “in ourselves” and loving someone “in God.” I.76-80 [25-26]).
Here is Augustine’s famous definition of love:
By love I mean the impulse of one’s mind to enjoy God on His own account and to enjoy oneself and one’s neighbor on account of God. … What love does to benefit itself is self-interest, and what it does to benefit a neighbor is known as kindness. And here self-interest comes first, because nobody can do good to another out of resources which he does not possess. The more the realm of lust is destroyed, the more the realm of love is increased. Augustine, On Christian Teaching, 3.37-38 
There is still an element of uncertainty here. I am saying that we enjoy a thing which we love for itself, and that we should enjoy only a thing by which we are made happy, but use everything else. God loves us [but] if he enjoys us, he stands in need of our goodness, which only a madman could assert; for all our goodness either comes from him or actually consists of him… So God does not enjoy us, but uses us. (If he neither enjoys nor uses us, then I fail to see how he can love us at all). Augustine, On Christian Teaching, 1.73-74 .
3. Augustine, The City of God against the Pagans, translated and edited by R.W. Dyson, Cambridge Texts in the History of Political Thought (New York, NK: Cambridge University Press, 2002).
Augustine thinks the chief folly of pagan philosophers is to seek the “Final Good” or “the Supreme Good” in something temporal such as the body, the soul, or virtue, and then seeking to achieve it “by their own efforts” (Augustine, The City of God, 19.4 ). He goes on to underscore the limitations of the best of pagan virtue and scrutinizes as absurd the notion that this life, with all its attendant miseries, can truly be called happy, concluding: “Let them no longer suppose that the Final and Supreme Good is something in which they may rejoice while in this mortal condition” (Ibid., 19.4 ). Even though Augustine puts great stress on how “we do not enjoy a present happiness,” yet he affirms that “it is in hope that we have been made happy” (ibid., Italics added). This paradox demonstrates that Augustine’s language is imprecise, and when he denies present happiness, he does not intend to rule out present delight or happiness altogether, but considers this life a sad prospect for ultimate happiness. There is no doubt that Augustine’s contemporary circumstances (read: the fall of Rome) as well as Scripture itself greatly shaped his concern to emphasize the temporal limitations of the “city of man.”