One of the best books on St. Augustine in the last 20 years is now in paperback, and therefore more affordable.
Serious students of late antiquity, of theology, of patristics, of politics.
Check out Robert Dodaro’s excellent, Christ and the Just Society in the Thought of Augustine.
Dodaro, an American, is the President of the Augustinianum, the Patristic Institute in Rome.
This is not a book on Augustine for beginners. If you are interested in a first look at the great Bishop of Hippo look at Serge Lancel’s Saint Augustine or else the old standby by Peter Brown Augustine of Hippo: A Biography (New Edition, with an Epilogue). Avoid James O’Donnell’s risible book as a waste of time and money. And you can’t go too wrong with Augustine (Past Masters) by Henry Chadwick, who died recently. RIP.