Kerstiens, Thom
Praeger , 1966
Buku Teks SO
ILO, 1974
Buku Teks SO
Esacove, Anne
Oxford University Press, 2016
eBooks
Nigel J. D. Lucas, Jayaprakasan Ambali, Enggene Chang, Marinette S. Forbes-Ricarte, Ram Manohar Shrestha
McGraw-Hill, 1987
Buku Referensi
edited by John Wilson Lewis
Stanford University Press, 1974
Buku Teks SO