Nainggolan, Poltak Partogi, 1963-
Yayasan Pustaka Obor Indonesia, 2020
Buku Teks
Artikel Jurnal
Korten, David C.
Yayasan Obor Indonesia, 2002
Buku Teks SO
Department of Foreign Affairs and Trade Canberra Australia
Pustaka Sinar Harapan , 1994
Buku Teks SO
Korten, David C.
Yayasan Obor Indonesia, 2001
Buku Teks SO