St. John Cassian
This is a classic spiritual text on the nature of sin and how to overcome it.
In this text St. Cassian begins with the most carnal of sins, gluttony, and proceeds to the most spiritual and hideous of all sins, pride. Along the way he unmasks the spirit of fornication, avarice, anger, sadness, acedia, and vainglory. He not only exposes these sins, but provides the believer inspired guidance on how to beat these sins and overcome them in this life. This text in the Philokalia has been cherished by believers for over a millennium.
On the Eight Vices