I'd like to see the whole Classics of Western Spirituality series (Paulist Press). A few volumes are less of interest to Christians than others (e.g., Early Islamic Mysticism), but the whole set would make a great bundle. Useful for those studying church history, spiritual theology, and in some cases homiletics or biblical studies as some of these volumes contain sermons.
I've marked what I think are probably the most important ones with an asterisk (*).
Abraham Isaac Kook: The Lights of Penitence, The Moral Principles, Lights of Holiness, Essays, Letters, and Poems
Abraham Miguel Cardozo: Selected Writings
Albert and Thomas: Selected Writings
Alphonsus de Liguori: Selected Writings
*Anchoritic Spirituality: Ancrene Wisse and Associated Works
Angela of Foligno: Selected Writings
Angelic Spirituality: Medieval Perspectives on the Ways of Angels
Angelus Silesius: The Cherubinic Wanderer
*Anglo-Saxon Spirituality: Selected Writings
*Apocalyptic Spirituality
*Athanasius: The Life of Antony and The Letter to Marcellinus
*Augustine of Hippo: Selected Writings
*Bernard of Clairvaux: Selected Works
Bérulle and the French School: Selected Writings
Birgitta of Sweden: Life and Selected Writings
*Bonaventure: The Soul's Journey into God, The Tree of Life, The Life of St. Francis
Cambridge Platonist Spirituality
Carthusian Spirituality: The Writings of Hugh of Balma and Guigo de Ponte
*Catherine of Genoa: Purgation and Purgatory, The Spiritual Dialogue
*Catherine of Siena: The Dialogue
*Celtic Spirituality
*Classic Midrash, The: Tannaitic Commentaries on the Bible
Devotio Moderna: Basic Writings
*Early Anabaptist Spirituality: Selected Writings
*Early Dominicans: Selected Writings
*The Early Kabbalah
Early Islamic Mysticism: Sufi, Qur'an, Mi'raj, Poetic and Theological Writings
Elijah Benamozegh: Israel and Humanity
Elisabeth of Schonau: The Complete Works
*Emanuel Swedenborg: The Universal Human and Soul-Body Interaction
*Ephrem the Syrian: Hymns
Fakhruddin Iraqi: Divine Flashes
*Francis and Clare: The Complete Works
*Francis de Sales, Jane de Chantal: Letters of Spiritual Direction
*Francisco de Osuna: The Third Spiritual Alphabet
*George Herbert: The Country Parson and the Temple
*The Germanica of Martin Luther
Gertrude of Helfta: The Herald of Divine Love
*Gregory of Nyssa: The Life of Moses
*Gregory Palamas: The Triads
*Hadewijch: The Complete Works
*Henry Suso: The Exemplar, with Two German Sermons
*Hildegard of Bingen: Scivias
Ibn ‘Abbad of Ronda: Letters on the Sufi Path
Ibn ‘Ata’ Illah/Kwaja Abdullah Ansari: The Book of Wisdom and Kwaja Abdullah Ansari, Intimate Conversations
Ibn Al’ Arabi: The Bezels of Wisdom
*Ignatius of Loyola: Spiritual Exercises and Selected Works
Isaiah Horowitz: The Generations of Adam
Jacob Boehme: The Way to Christ
Jacopone da Todi: Lauds
*Jean Gerson: Early Works
*Jeremy Taylor: Selected Works
Jewish Mystical Autobiographies: Book of Visions and Book of Secrets
*Johann Arndt: True Christianity
*Johannes Tauler: Sermons
*John and Charles Wesley: Selected Prayers, Hymns, Journal Notes, Sermons, Letters and Treatises
*John Baptist de La Salle: The Spirituality of Christian Education
*John Calvin: Writings on Pastoral Piety
*John Cassian: Conferences
*John Climacus: The Ladder of Divine Ascent
*John Comenius: The Labyrinth of the World and The Paradise of the Heart
*John Donne: Selections from Divine Poems, Sermons, Devotions and Prayers
*John Henry Newman: Selected Sermons
*John of the Cross: Selected Writings
*John Ruusbroec: The Spiritual Espousals, The Sparkling Stones, and Other Works
*Julian of Norwich: Showings
Knowledge of God in Classical Sufism: Foundations of Islamic Mystical Theology
*Luis de León: The Names of Christ
Margaret Ebner: Major Works
*Marguerite Porete: The Mirror of Simple Souls
Maria Maddalena de' Pazzi: Selected Revelations
*Maximus the Confessor: Selected Writings
*Mechthild of Magdeburg: The Flowing Light of the Godhead
*Meister Eckhart: Teacher and Preacher
*Meister Eckhart: The Essential Sermons, Commentaries, Treatises and Defense
Menahem Nahum of Chernobyl: Upright Practices, The Light of the Eyes
Nahman of Bratslav: The Tales
Native Meso-American Spirituality
Native North American Spirituality of the Eastern Woodlands: Sacred Myths, Dreams, Visions, Speeches, Healing Formulas, Rituals
*Nicholas of Cusa: Selected Spiritual Writings
Nicodemos of the Holy Mountain: A Handbook of Spiritual Counsel
Nil Sorsky: The Complete Writings
Nizam Ad-Din Awliya: Morals for the Heart
*Origen: Selected Writings
*Philo of Alexandria: The Contemplative Life, Giants and Selections
*The Pietists: Selected Writings
*Pseudo Dionysius: The Complete Works
*Pseudo-Macarius: The Fifty Spiritual Homilies and The Great Letter
*Quaker Spirituality: Selected Writings
Rabbinic Stories
*Richard of St. Victor: The Book of the Patriarchs, The Mystical Ark, Book Three of the Trinity
*Richard Rolle: The English Works
*Robert Bellarmine: Spiritual Writings
Safed Spirituality: Rules of Mystical Piety, The Beginning of Wisdom
*Shakers, The: Two Centuries of Spiritual Reflection
Sharafuddin Maneri: The Hundred Letters
*Symeon the New Theologian: The Discourses
*The Talmud: Selected Writings
*Teresa of Avila: The Interior Castle
The Pilgrim's Tale
The Pursuit of Wisdom and Other Works by the Author of the Cloud of Unknowing
The Spirituality of the German Awakening
Theatine Spirituality: Selected Writings
Umar Ibn al-Farid: Sufi Verse, Saintly Life
Valentin Weigel: Selected Spiritual Writings
Vincent de Paul and Louise de Marillac: Rules, Conferences, and Writings
*Walter Hilton: The Scale of Perfection
*William Law: A Serious Call to a Devout and Holy Life, The Spirit of Love
Zohar: The Book of Enlightenment