The best books about Orthodoxy are books by Orthodox bishops. What one discovers is that Orthodoxy is not such a radical departure from the faith that some people claim, but that it is a departure from the faith in certain areas, especially in the doctrines of original sin, marriage, and (in spite of their great devotions to her) the Blessed Virgin.
I didn't mention the "Filioque" issue because, in spite of this being the most known controversy between Orthodox and Catholic, I still am unable to understand the problem. I've read explanations of what procession of the Holy Ghost by both Orthodox and Catholic theologians and I don't see a real difference and, frankly, don't understand how that one word caused such a ruckus in the 11th Century.