The Imitation of Christ by Thomas à Kempis.  
The Imitation of Christ is widely considered one of the greatest manuals of devotion in Christianity. The life of Christ is presented as the highest study possible to a mortal, as Jesus’ teachings far excel all the teachings of the saints. The book gives counsel to read the scriptures, statements about the uses of adversity, advice for submission to authority, warnings against temptation and how to resist it, reflections about death and the judgment, meditations upon the oblation of Christ, and admonitions to flee the vanities of the world.