The Orthodox Church is best experienced in the Divine Liturgy, where Christ is in our midst. But any one of the following introductions is helpful.
Fr Anthony Canaris, Introducing the Orthodox Church; Its faith and life (Light & life);
Timothy Ware (Bishop Kallistos), The Orthodox Church (New Edition) (Penguin);
Fr Thomas Hopko, The Orthodox Faith in 4 volumes (The Orthodox Church in America)
Daniel Daly, The Choice of Orthodoxy: The Church-One, Holy, Catholic, and Apostolic
Clark Carlton, The Faith: Understanding Orthodox Christianity. An orthodox Catechism