Kerstiens, Thom
Praeger , 1966
Buku Teks
Wint, Guy
Penguin Books, 1955
Buku Teks
Esacove, Anne
Oxford University Press, 2016
eBooks
ILO, 1974
Buku Teks
Nigel J. D. Lucas, Jayaprakasan Ambali, Enggene Chang, Marinette S. Forbes-Ricarte, Ram Manohar Shrestha
McGraw-Hill, 1987
Buku Referensi