The City of God, by Saint Augustine.
This tome is considered a classic work of theology, and as I read it I understand why. So much of what we think of as orthodoxy derives directly from this work, and is really the template for Christian apologetics. In this case, much of what Saint Augustine is arguing against is the Roman gods (Zeus and that bunch), and so is not directly applicable today. But the idea of taking on the "gods of the age" and comparing the truth of their claims to the truth of Jesus taught the Church how to face the claims of the false gods that have arisen to challenge Christ in the 1600 years since.
The work is much longer and more academic than The Confessions, the other great literary contribution of Saint Augustine. That is the template of the spiritual biography, and is probably the place to start with Saint Augustine.
I am not recommending that others read this (it is long, and some parts are not applicable and others are dry), but it is important work, and we should know that it is an important work.