Father Robert Barron on Spiritual Classics Every Catholic Should Read

Origen: De Principiis
Irenaeus: Adversus Haereses
Anything by Augustine
Sermons of Chrysostom
Saint Jerome's Biblical Commentary
Maximus the Confessor: anything
Gregory of Nyssa: anything
Anselm: Why God Became a Man" & Proslogion
Aquinas: Summa Theologiae, De Ente et Essentia, Bible commentaries by him, Golden Chain, Summa Contra Gentiles

Bonaventure: The Mind's Road to God
John Henry Newman: Apologia Pro Vita Sua & An Essay on the Development of Christian Doctrine
GK Chesterton: anything
Ignatius of Loyola: spiritual exercises
St. John of the Cross: anything
Thomas Merton: Seven Story Mountain, New Seeds of Contemplation
...that's just "the surface of the surface"