St. Augustine of Hippo 
The Doctrinal Treatises Of St. Augustine [EPUB ebook] 

The most weighty of the doctrinal treatises is that on the Holy Trinity. The Latin original (De Trinitate contra Arianos libri quindecim), is contained in the 8th volume of the Benedictine edition. It is the most elaborate, and probably also the ablest and profoundest patristic discussion of this central doctrine of the Christian religion, unless we except the Orations against the Arians, by Athanasius, ‘the Father of Orthodoxy, ‘ who devoted his life to the defense of the Divinity of Christ. Augustine, owing to his defective knowledge of Greek, wrote his work independently of the previous treatises of the Eastern Church on that subject. He bestowed more time and care upon it than on any other book, except the City of God.
Besides this treatise the following works are included:
The Enchiridion, or On Faith, Hope and Love
On the Catechising of the Uninstructed
Treatise on Faith and the Creed
Concerning Faith of Things not Seen
On the Profit of Believing
On the Creed