edited by Peter Moore, R.J. Berry and A. Hallam
Grolier International, 1993
Buku Referensi
edited by John L. Gittleman
Comstock , 1989
Buku Teks
Richardson, W. Norman
Macmillan, 1976
Buku Teks
Jiggins, Chris D.
Oxford University Press, 2017
eBooks
Haugen, Einar
Stanford University Press, 1972
Buku Teks SO