Here are St. John of the Cross (Mystical Doctor of the Church)'s free online books:
Ascent of Mount Carmel:
ccel.org/ccel/john_cross/ascent.ii.vii.html
Dark Night of the Soul:
ccel.org/ccel/john_cross/dark_night.html
Spiritual Canticle:
karmel.at/ics/john/cn.html
The Living Flame of Love:
karmel.at/ics/john/fl.html
Here are St. Teresa of Avila (Doctor of the Church, Founder of the Discalced Carmelites and under the spiritual direction of St. John of the Cross)'s free online works:
The Life of St. Teresa of Avila:
catholicfirst.com/TheFaith/CatholicClassics/StTeresa/life/teresaofavila.cfm
The Way of Perfection
catholicfirst.com/TheFaith/CatholicClassics/StTeresa/way/wayofperfection.cfm
Interior Castle:
sacred-texts.com/chr/tic/index.htm
S. Therese of Liseux is also a Doctor of the Church and Carmelite, but maybe she is more an expert on humility and service to God and others, and missionary work than on Mystical Theology
Autobiography of St. Therese of Liseux: The Story of a Soul
gutenberg.org/etext/16772
The Teaching of St. Therese of Liseux on Purgatory:
franciscan-sfo.org/ap/litfwrpu.htm
*Hail Mary, full of grace, the Lord is with Thee. Blessed art Thou among women and blessed is the fruit of Thy womb, Jesus. Holy Mary, Mother of God, pray for us sinners now and at the hour of our death. Amen. *
Come Holy Spirit, come by means of the powerful intercession of the Immaculate Heart of Mary, your well beloved spouse. Come Holy Spirit, come by means of the powerful intercession of the Immaculate Heart of Mary, your well beloved spouse. Come Holy Spirit, come by means of the powerful intercession of the Immaculate Heart of Mary, your well beloved spouse.
For the sake of his sorrowful passion, have mercy on us and on the whole world! For the sake of his sorrowful passion, have mercy on us and on the whole world! For the sake of his sorrowful passion, have mercy on us and on the whole world!
O Mary, conceived without sin, pray for us who have recourse to thee!
Sorrowful and Immaculate Heart of Mary, pray for us!
*Hail Mary, full of grace, the Lord is with Thee. Blessed art Thou among women and blessed is the fruit of Thy womb, Jesus. Holy Mary, Mother of God, pray for us sinners now and at the hour of our death. Amen. *