Hmm, so far as I can remember, here are the ones that I got the most out of when I was Orthodox...
The Works of Gregory the Theologian, by Gregory the Theologian - IMO the best theologian Christianity has produced.
The Sayings of the Desert Fathers, by Anthony and Ward - Gives you lots of things to think about concerning humility, judgmentalism, etc.
Deification in Christ: Orthodox Perspectives on the Nature of the Human Person, by Panayiotis Nellas - Best book I've read on anthropology from an Orthodox Christian perspective.
Orthodox Faith and Life in Christ, by Justin Popovich - Engaging collection of essays, outlining a traditionalist view of the world.
Orthodoxy and Ecumenism, by Justin Popovich - The best work on Orthodox ecclesiology that I know of.
The Life in Christ, by Nicholas Cabasilas - Lots of good thoughts from an Orthodox perspective on the sacraments, salvation, free-will, and other subjects.
Women and Men in the Early Church: The Full Views of St. John Chrysostom, by David Ford - Good book on sexuality, and the role of men and women in various contexts.
Scripture and Tradition, by Chrysostomos and Auxentios - One of the best summations of the Orthodox position on scripture and tradition.
In the Image and Likeness of God, by Vladimir Lossky - Has a lot of interesting things to think about if you are Orthodox.
The Works of John Cassian, by John Cassian (and others) - Great look at early Christian monasticism, and also a synergetic view of salvation.
St Gregory of Nazianzus: An Intellectual Biography, by John Anthony McGuckin - Since I consider Gregory the greatest Christian theologian, it only stands to reason that I'd consider an exhaustive and engaging biography of his to be a great book.
Way of the Ascetics: The Ancient Tradition of Discipline and Inner Growth, Tito Colliander - Lots of good practical thoughts if you're Orthodox.
How Are We Saved?: The Understanding of Salvation in the Orthodox Tradition, Bishop Kallistos - Good introduction to the Orthodox view of salvation.
The Collected Works of Georges Florovsky; Volume One: Bible, Church, Tradition: An Eastern Orthodox View, by Georges Florovsky - Good stuff on the Scripture and Tradition, and the intertwining of the two.
The Desert a City: An Introduction to the Study of Egyptian and Palestian Monasticism Under the Christian Empire, by Derwas Chitty - Good, short history of the early Christian monastic movement(s).