Theologian’s Library’s most popular post: a recommended reading list from David Bentley Hart. Happy reading!
I assume you want basic guides to metaphysics to begin with. Richard Taylor’s Metaphysics (4th edition) is a good introduction to certain classic questions. Two volumes by E. L. Mascall–He Who Is and Existence and Analogy–are excellent guides to Christian metaphysics in the West, as is W. Norris Clarke’s The One and the Many. Avoid Peter van Inwagen: he’s brilliant at confusing things. Perhaps William Hasker’s Metaphysics is good, but I haven’t read it.
As for theology, always start with the fathers: Gregory of Nyssa’s On the Making of Man and On the Soul and Resurrection, Ps-Dionysius Complete Works (a short volume), Cyril of Alexandria’s On the Unity of Christ, Athanasius On the Incarnation, St Isaac of Ninevah’s treatises (especially the “Second Volume”), Maximus the Confessor’s Chapters on Love and the SVS volume “The Cosmic Mystery of Christ.” Don’t bother with Augustine until later, because for all his genius he got so much wrong (because he couldn’t read Greek, in large part). For mediaeval and early modern theology, Symeon the New Theologian’s Mystical Discourses (or whatever it’s called in English), Bonaventure’s Journey of the Mind into God, and the Paulist Press volume of Nicholas of Cusa. Thomas Traherne’s Centuries and all three volumes of George MacDonald’s Unspoken Sermons are indispensable masterpieces.
As for modern theologians, Bulgakov’s Bride of the Lamb, Hans Urs von Balthasar’s Glory of the Lord, Lossky’s Mystical Theology of the Eastern Church, Schmemann’s For the Life of the World, part IV of Karl Barth’s Church Dogmatics, Henri de Lubac’s Supernatural (currently being translated I believe, but if you read French go ahead), Rowan Williams’ Resurrection (2nd edition).
I hope this helps,