Soule, George, author
Mentor Book, 1952
Buku Teks
Hovey, Craig, author
Prenada, 2009
Buku Teks
Simanjuntak, Ekky Gompa, author
The Ary Suta Center, 2019
Artikel Jurnal
Soule, George, author
Pustaka Rakjat, 1952
Buku Teks
Maxwell, John C., author
Menuju Insan Cemerlang (MIC), 2015
Buku Teks