Interior Castle, by St. Teresa of Avila, is one of the saint's writings that is filled with spiritual insight, charm, and breathtaking closeness to God. Her castle is the God within, and there are outward mansions that must be penetrated to get to this inner treasure. Each mansion is a deepening of one's knowledge of God, and a step further away from one's capture by externality and the world. She describes these inward steps, and leads one onward to union with God (or God carries one onward - at first it is our efforts, than it is God's). Beautiful and illuminating.