Encyclopaedia Britannica, 1981
Buku Referensi
Encyclopaedia Britannica, 1981
Buku Referensi
Parrot, Martin, author
Cambridge University Press, 2003
Buku Teks
Crystal, David, 1941-, author
Cambridge University Press , 2007
Buku Teks
Koneru, Aruna, author
Tata McGraw-Hill, 2011
Buku Teks