OK, here are 5 that have been very helpful to me as a Protestant convert to Orthodoxy. Mind you that I am Ethiopian Orthodox- (OO).
1. "The Ethiopian Tewahedo Church"
by Archbishop Abuna Yeshaq
In my opinion, this book is the ultimate guide to the Ethiopian Orthodox Church. Thanks be to God for Archbishop Yesehaq, who was sent by His Majesty Haile Selassie I to the Western Hemisphere to establish the true and ancient Christian Faith. This is the man who baptized Bob Marley.
2. "Eastern Orthodox Christianity: A Western Perspective"
by Daniel B. Clendenin
This book is written by a Protestant, but it is essentially an apologetic and explanation of Orthodoxy for those who know little about it. This book has really helped me to understand the why's and what's of Orthodox practices, customs, and worship. It is an excellent guide for those who are curious about Orthodoxy, or for Protestant converts such as myself. My only criticism is that it doesn't mention anything about the Non-Chalcedonian Churches.
3. "The Orthodox Church"
by Kallistos (Timothy) Ware
This is another great guide to Orthodoxy. Very readable. The index is especialy helpful. If you want to know about a certain Orthodox doctrine or practice, you can easily look it up and find a clear explanation.
4. "The Ethiopian Synaxarium"
This book is the daily lives of the Saints. You can read it and download it on line here:http://www.stmichaeleoc.org/Synaxarium/Archive.htm
5. "The Way of a Pilgrim"
Written anonymously, this short work is considered to be the classic Russian Orthodox book on prayer. Easy to read, but full of spiritual depth that can never be exhausted.
6. "Father Seraphim Rose: His Life and Works"
by Heirmonk Damascene
I'm only about half way through this tome, but I am enjoying it. The first 200 pages were really good, but then it started to drag a bit. But a friend of mine has assured me that it really picks back up after you get past the half-way point. I guess it depends on what you like. For example, I love reading about people's spiritual journeys and their philosophical development. but I get a bit bogged down when biogrpahers feel the need to include every influential person in the subject's life. Sometimes it seems like there are too many chapters devoted to biographies of the individuals that influenced Father Seraphim Rose, rather than sticking to the life of Seraphim rose himself. I would prefer to read more of Rose's actual writings and his philosophical/theological transformation. Another disappointment is the omission of many of the details of Seraphim Rose's trials and struggles before his conversion. This is a long book, but overall I think it is very interesting and worth the read.
Hope that helps.