I would recommend "The Vision of God" by Vladimir Lossky. It is less than 200 pages and will serve as a good introduction to the development of theology in the early church in the eastern part of the Roman Empire. I must admit I couldn't make much sense of it the first time I read it, mainly because it didn't coincide with the previous Neoplatonic interpretations of Christianity that I had read. I read it again 6 months later and discovered that I had a very different understanding of Christianity because of this book.
"The Mystical Theology of the Eastern Church" is one of the best books ever written.