St. Teresa of Ávila 
The Interior Castle [EPUB ebook] 

Saint Teresa of Ávila (1515–1582), was a prominent Spanish mystic, Roman Catholic saint, Carmelite nun and author during the Counter Reformation, and theologian of contemplative life through mental prayer. She was a reformer in the Carmelite Order of her time and the movement she initiated, later joined by Saint John of the Cross, eventually led to the establishment of the Discalced Carmelites, though neither she nor John were alive when the two orders separated.