On the Incarnation: Saint Athanasius The Great of Alexandria (Popular Patrictics Series, 44b)

By any standard, this is a classic of Christian theology.

Composed by St Athanasius in the fourth century, it expounds with simplicity the theological vision defended at the councils of Nicaea and Constantinople: that the Son of God himself became "fully human, so that we might become god." Its influence on all Christian theology thereafter, East and West, ensures its place as one of the few "must read" books for all who want to know more about the Christian faith.