For the reader coming to Augustine for the first time, the following essays are meant to introduce his life and thought. They were written in the early 1980s and published as Augustine in the Twayne World Authors series in 1985. The best book-length studies of Augustine's life are (recent and brief and very illuminating) Garry Wills, Augustine (Penguin, 1999) and (the classic study, soon to appear in a new edition with additional chapters updating the original) Peter Brown, Augustine of Hippo (Berkeley, 1967). I have continued to worry about the way we talk about Augustine's life and have written some additional essays of a more specialized nature on those topics.
For Augustine's influence in later ages, see my "The Authority of Augustine", the 1991 St. Augustine Lecture at Villanova University.
The Stanford Encyclopedia of Philosophy has a thorough entry on Augustine by Michael Mendelson, emphasizing philosophical issues and contribution. I have also contributed the new entry on Augustine for the Encyclopedia Britannica