- Ostsyrisches Christentum: Untersuchungen zu Christologie, Ekklesiologie und zu den ökumenischen Dialogen der Assyrischen Kirche des Ostens [East Syriac Christianity: Studies on Christology, Ecclesiology and the Ecumenical Dialogues of the Assyrian Church of the East], Vienna: LIT, 2004.
- Syriac Churches encountering Islam: Past experiences and future perspectives, Piscataway, NJ: Gorgias Press, 2010 (Pro Oriente Studies in Syriac Tradition, 1).
- (with Li Tang), From the Oxus River to the Chinese Shores: Studies on East Syriac Christianity in China and Central Asia, Vienna: LIT, 2013 (orientalia – patristica – oecumenica, 5).
- (with Li Tang), Hidden Treasures and Intercultural Encounters: Studies on East Syriac Christianity in China and Central Asia, Vienna: LIT, 2009 (orientalia – patristica – oecumenica, 1); 2nd ed., 2014.
- (with Li Tang), Winds of Jingjiao: Studies on Syriac Christianity in China and Central Asia, Vienna: LIT, 2016 (orientalia – patristica – oecumenica, 9).
- (with Li Tang), Artifact, Text, Context: Studies on Syriac Christianity in China and Central Asia, Vienna: LIT, 2020 (orientalia – patristica – oecumenica, 17).
- Towards a Culture of Co-Existence in Pluralistic Societies: The Middle East and India, Piscataway, NJ: Gorgias Press, 2020 (Pro Oriente Studies in Syriac Tradition, 4).
- (with Li Tang), Silk Road Traces: Studies on Syriac Christianity in China and Central Asia, Vienna: LIT, 2022 (orientalia – patristica – oecumenica, 21).
- (with Wilhelm Baum), Die Apostolische Kirche des Ostens: Geschichte der sogenannten Nestorianer, 2000 (Einführungen in das orientalische Christentum, 1); English ed., The Church of the East: A concise history, London and New York: RoutledgeCurzon, 2003; Romanian ed., Biserica Asiriana a Rasaritului: O scurta istorie a crestinismului siro-oriental, Iasi: Doxologia, 2021; Persian ed., Tārīkh-e Mukhtaṣar-e Kelīsā-ye Sharq, Pardisan, Qom: University of Religions and Denominations Publications, 2023).
- (with Andreas Schmoller), Identity and Witness: Syriac Christians of the Middle East and India between homeland and global presence, Piscataway, NJ: Gorgias Press, 2023 (Pro Oriente Studies in Syriac Tradition, 5).
- (with Christian Lange et al.), Die katholischen Ostkirchen [The Catholic Eastern Churches], Freiburg im Breisgau: Herder, 2024.