The Dark Night of the Soul . John of the Cross

This book tells about the mystical journey of the soul toward the union with God. This journey is made in darkness since we don't know anything of God, the Light, at the end of the trip. The journey comprises two critical phases of purgation: the purgation of senses and the purgation of spirit. These two phases make the first part of the spiritual journey. After the purgation, there comes illumination and then union.