The Interior Castle. A 16th-Century Nun Maps the Soul as a Castle With Seven Rooms Leading to God

Teresa of Avila wrote The Interior Castle in a matter of weeks, describing the soul as a crystal castle made of seven concentric "mansions, " each one a stage closer to union with God. It's part mystical vision, part practical guide for anyone trying to go deeper in prayer than the outer rooms - grounded, specific, and startlingly honest about the doubts and dry spells along the way, not just the moments of ecstasy.
Written by a woman who reformed the Carmelite order and was later declared a Doctor of the Church, this remains one of the most influential texts in the history of Christian mysticism - read as literature, spiritual guide, or both.