Augustine by Henry Chadwick

Saint Augustine was a churchman who wrote extensively for the Roman Catholic Church around the year 400 A.D., and that makes him an "ancient" writer, like the Roman philosophers. His words were so influential that many of the policies he helped to set are still in place here in the 21st century. 

The author gives a good idea of the hotbed of activity that was the Catholic Church in the 5th Century. But if you want to go into greater depth on certain subjects, like the Trinity or how the Church's ideas on abortion were formed, it would be better to read elsewhere, as this book is a very erudite overview.