I think getting a grounding in the fundamentals of belief and practice makes sense before going on to more lofty spiritual literature, but in general I wouldn't echo warnings about with reading the Philokalia, Sayings of the Desert Fathers, etc.*, supposing you are able to approach them with a healthy or balanced point of view. After all, one could also take many passages in Scripture and go sort of crazy with them, or get driven into despair or delusion. I think the important thing is moderation and sobriety, and that we be sure that any zeal comes from our experience with God's grace, and not our own cravings or personality quirks. This is not to say that I disagree per se with the above warnings, which I think are good to hear; I just wouldn't go as far.
Anyway, my favorites/recommendations (including for apologetic purposes) would probably be:
- The Orthodox Way, by Met. Kallistos Ware. A good overview of the general direction of Orthodox spirituality.
- The Way of the Ascetics, by Tito Colliander. Along the lines of some of what is in the Philokalia, but written for lay people in the world.
- The Christian Tradition: A History of the Development of Doctrine, by Jaroslav Pelikan. A good 5-volume history of Christianity written in a generally even-handed manner.
- Father Arseny, 1893-1973: Priest, Prisoner, Spiritual Father. An inspirational story of a modern saint.
- The Orthodox Church: An Introduction to its History, Doctrine, and Spiritual Culture, Fr. John Anthony McGuckin. An intro like The Orthodox Church by Met. Kallistos, but less popular level.
- You Are Peter: An Orthodox Reflection on the Exercise of Papal Primacy, by Olivier Clement. An irenic if brief look at the topic by an Orthodox Christian.
- The Primacy of Peter: Essays in Ecclesiology and the Early Church, by John Meyendorff. I agree with the above recommendation by Sam G.
The two Whelton books are also options. I find them a bit too polemical and... . . well I'll just leave it at that, it's been too long since I read them to say much with accuracy.
Another book that I have on a list to buy, but haven't read yet, is Petrine Ministry: Catholics And Orthodox In Dialogue, edited by Walter Cardinal Kasper, which includes presentations by both Catholic and Orthodox scholars.
Other things to consider for reading material, not dealing with arguments from an apologetic perspective, but rather a historical one, is how the Church in various places dealt with divisions and attempts at unification, like the Meletian schism, the Council of Florence, The Photian schism, etc.
*On rereading the thread I see that no one here spoke of caution regarding the Sayings of the Desert Fathers, I must have been thinking of a previous thread with that one in particular.