Botha, Rudolf P, author
Cambridge University Press, 2016
Buku Teks
Alfonso, Anthony, author
Sophia University LL Center of Applied Linguistics, 1980
Buku Teks
University of Bath, 1985
Buku Referensi
Terry, Patrick D., author
Addison-Wesley, 1986
Buku Teks
Fishman, Joshua A., author
Newbury House Publishers, 1972
Buku Teks