Reader's Digest Association.
Reader's Digest Association, 1968
Buku Referensi
Nelson, Hank
Penguin Books, 1974
Buku Teks
Durack, Elizabeth
Ure Smith , 1970
Buku Teks
Fakultas Ilmu Sosial dan Ilmu Politik Universitas Indonesia, 1992
UI - Skripsi (Membership)
Leach, James, author
Berghahn Books, 2004
Buku Teks