The thinking of St Augustine of Hippo, a Doctor of the Church, is so influential, says Pope Benedict XVI, that he continues to leave a “deep mark on the cultural life of the West and on the whole world”.
Much of his philosophy and theology is contained in an extensive book called The City of God, but his spiritual autobiography, Confessions, has been the most popular of his works down the centuries. This classic forms an excellent introduction to Augustine, whose feast was celebrated on August 28, and demonstrates the intelligence, wit and sheer brilliance of his prose. There is also a film of Confessions which can be viewed on YouTube.
For a broader introduction, the books of Henry Chadwick can be recommended, and even more so the five general audiences given about the saint and his writings by Benedict XVI in January and February 2008, which are available on the Vatican website.
In the last of these, Benedict says that his first encyclical, Deus Caritas Est, published in 2005, owed much “to Augustine’s thought”. So if you want to understand Augustine, this document is as good a starting place as any.