Apostolic Christian Church of America (2000) Zion’s Harp, Goodfield, IL:  Apostolic Christian Bookstore

Apostolic Christian Church Nazarene (2005), Apostolic Christian Hymnal, Syracuse, NY:  Apostolic Christian Nazarene Publishing Company

Bekic, Cedomir (2018) Email correspondence, Ontario, Canada.

Enhle, Kurt and Jane (2012) Heritage of Faith: A History Of The Church With An Apostolic Christian Focus, Fairbury, IL:  Heritage Center Foundation
 
Enhle, Willis (2019) Email correspondence, Illinois, USA.
 
Gemeinschaft evangelisch Taufgesinnter (Nazarener) (1956) Neue Zionsharfe, Zurich, Switzerland:  Karl Schippert & Co.
 
Igic, Mark (2017) Email correspondence, California, USA.
 
Klopfenstein, Perry A., A Treasure of Praise:  A History of the Zion’s Harp Hymnal, Eureka, IL:  Apostolic Christian Publications
 
Kniesel, Armin (2019) Email correspondence, Heilbronn, Germany

Miller, Eric, Zion's Harp In Verse, Sterling, OH

Remenyi, Csaba (2018) Email correspondence, Hódmezővásárhely, Hungary.
 
Steffen, Nathan (2016) Interview, Illinois, USA.
 
Virkler, Dennis (2012), Zion’s Harp CD Pamphlets, Carlock, IL:  Rejoice Distribution
 
Numerous others, sharing small pieces of information at a time...