Thomas a Kempis' Imitation of Christ, below on Christian Classics.
http://www.ccel.org/ccel/kempis/imitation
From the Christian Classics website--
For five hundred years, this gentle book, filled with the spirit of the love of God, has brought understanding and comfort to millions of readers in over fifty languages, and provided them with a source of heart-felt personal prayer. These meditations on the life and teachings of Jesus, written in times even more troubled and dangerous than our own, have become second only to the Bible as a guide and inspiration.
