A while back I was asked by an Orthodox priest if I could provide him some references for Joseph Cardinal Ratzinger's writings about ecumenism. In light of the Holy Father's current trip to Turkey, here is the list I came up with. It is undoubtedly incomplete, but may be helpful for those interested in reading more in this area:
1). Principles of Catholic Theology (Ignatius, 1987. Orig. German, 1982) contains a major section (pages 193-314) on ecumenism, including:
I. General Orientation with Regard to the Ecumenical Dispute about the Formal Principle of Faith. A large part of this directly addresses Catholic-Orthodox relations. This was originally a lecture delivered in 1976.
II. The Key Question in the Catholic-Protestant Dispute: Tradition and Successio Apostolica
III. Catholicity as the Formal Structure of Christianity
2). The Meaning of Christian Brotherhood (Ignatius , 1993. Orig. German edition, 1960). This is an early synthesis of Ratzinger's ecumenical thought, with a strong emphasis on biblical exegesis.
3). Called to Communion: Understanding the Church Today (Ignatius, 1996. Orig. German, 1991). Ratzinger's ecclesiological vision, with a major section on the "Primacy of Peter" and Petrine office.
4). Pilgrim Fellowship of Faith: The Church as Communion (Ignatius, 2005. Orig. German, 2002). A lot of great material on ecumenism here. Includes an exchange of letters between Metropolitan Damaskinos and Ratzinger (pp 217-241), as well as "On the Ecumenical Situation" (pp 253-269). Recommended!
Two books that address inter-religious dialogue and relationships:
1). Many Religions, One Covenant: Israel, the Church, and the World (Ignatius, 1999. Orig. German, 1998). Focus on Judaism and shared heritage.
2). Truth and Tolerance: Christian Belief and World Religions (Ignatius, 2004. Orig. German, 2003). A nuanced critique of relativism, religious indifferentism, and pluralism.