Universitas Indonesia, 2003
Buku Teks
edited by T. Marzuki Jacob
Universitas Sumatera Utara, 1994
Buku Referensi
Mayhew, Lewis B.
Jossey-Bass, 1973
Buku Teks
Her Majesty's Stationery Office, 1987
Buku Teks
edited by Maria Slowey, David Watson
Open University Press, 2003
Buku Teks