What books were the most formative for you as an orthodox person?
In terms of ancient Patristic material: hymns of St. Ephrem the Syrian and certain other Syriac-speaking fathers, The Sayings of the Desert Fathers, the Panarion of St. Epiphanios of Palamis (Cyprus) and Against Heresies by St. Irenaeus of Lyons, and importantly everything written by St. Athanasius, who I particularly love, for example, On the Incarnation and The Life of Anthony (I particularly loved that). Also St. Isaac the Syrian and the Fount of Knowledge of St. John of Damascus. But above all else the liturgical texts.
Among more recent material, the Philokalia collection of later Greek Patristic writings by the 18th century Athonite monk St. Nicodemus the Hagiorite and St. Macarius of Corinth, translated by Metropolitan Kallistos Ware and Mother Mary, a Greek Orthodox nun, may their memories be eternal, also, The Orthodox Church and The Orthodox Way by Metropolitan Kallistos (who was very loving towards our Oriental Orthodox friends), and Orthodox Dogmatic Theology by Protopresbyter Michael Pomazansky, translated by Fr. Seraphim Rose, may their memories be eternal.
Fr. Seraphim Rose (not to be confused with his patron St. Serafim of Sarov) latter wrote Orthodoxy and the Religion of the Future which accurately foresaw the dangers of the Charismatic movement, Pentecostalism, interest in Hindu and Buddhist spirituality, and of UFO cults - it was written in the mid 1970s and just over 20 years later a notorious event occurred in the US where members of a small UFO cult formed by an apostate Christian seminarian gone total heretic did the unthinkable and ended their beliefs on the basis an orbiting UFO would transport their souls aboard. And Pentecostalism and the Charismatic movement have continued to grow. He also wrote a book on Nihilism which was rather good, but its a scholarly text, a brilliant indictment of the philosophical underpinnings of secularism and communism, suitable for someone with advanced English skills but I don’t think its available in translation (if it is, I would guess it would be in Russian, possibly French).
Finally, a 19th century Russian Orthodox bishop, St. Ignatius Brianchaninov, wrote two splendid books: the Arena, written for monastics but of general interest, and On The Prayer of Jesus, specifically for the laity.