There's no real easy way to answer this. Orthodoxy is a bottomless well of wisdom and beauty and it ultimately depends on what you're looking for. Dogmas? Christology? Ecclesiology? Spirituality? All would present you with different set of books to delve into. I compiled a large amount of free sources of audio for recent catechumen in the thread "Christmated into the Church, what to expect now?", there's a wealth of information there for you to check out. As for introductory books, I'm not all that sure as I skipped right past most of that and jumped into the writings of the saints and just began piecing the whole thing together. That said, I'll provide you a short(ish) list of books to consider, which cover a wide array of topics:Orthodoxy is the one branch of Christianity that I don't know much about. What are some good books or websites to begin learning? Also, for those who are Orthodox, would you say it's a fair assessment that it is the branch least tainted by humanity? Does that make sense? In order to be Catholic, one must believe in the infallibility of the Pope... in order to be Protestant, one must accept man's teaching on various denominational doctrines. Is Orthodoxy freer of such things?
1. Rock and Sand - Fr. Josiah Trenham (An Orthodox perspective on Protestantism - a good place to start if that is your foundation)
a) Two Paths: Orthodoxy and Catholicism - Dr Michael Whelton (A good place to start if Catholicism is your foundation)
2. The Way of the Pilgrim - Unknown Author (Beautiful text on Orthodox Spirituality - Audiobook can be found on youtube)
3. Theosis: The True Purpose of Human Life - Archimandrite George of Mount Athos (Audiobook also found on youtube)
4. Byzantine Theology - John Meyendorff
5. The Mystical Theology of the Eastern Church - Vladimir Lossky
6. On the Cosmic Mystery of Jesus Christ - St. Maximus the Confessor
7. Unseen Warfare - St. Nicodemus of the Holy Mountain
8. The Path to Salvation / The Spiritual Life - St. Theophan the Recluse
9. Deification of Man - Georgios Mantzaridis
10. Wisdom of the Desert: Sayings from the Desert Fathers - Thomas Merton
11. Wounded by Love - St. Porhpyrios
12. Saint Paisios of Mount Athos - Hieromonk Isaac
Outside of # 4 none of those are really "introductory" books to Orthodoxy, but honestly most of those types of books are kind of boring and can easily be replaced with various lecture series found online. The writings of the saints, however, kindles your heart and can bring you to tears, while also bestowing you with wisdom that the other books simply lack.