There are, of course, the Church Histories written in the Early Church which, of course, show that the early Church was not Roman Catholic or Protestant but Orthodox. Here are some of them:
Eusebius of Caesarea - Ecclesiastical Histories
Sozomen - Ecclesiastical Histories
Theodoret - Ecclesiastical Histories
Evagrius Scholasticus - Ecclesiastical Histories
Then you have the more modern ones.
Aristeides Papadakis - The Christian East and the Rise of the Papacy
Aristeides Papadakis - Crisis in Byzantium
Andrew Louth - Greek East and Latin West
Another book I really recommend:
Vladimir Lossky - The Mystical Theology of the Eastern Church
There are a lot more, of course, but these would be a nice start.