Walters, Morgan L
N.Y.Holt Rinehart and Winston,inc, 1966
Buku Teks
Longman, 1984
Buku Referensi
edited by Henry Cecil
George Routledge & Sons, 1946
Buku Referensi
Longman, 1992
Buku Referensi
editor in chief John Sinclair
Collins, 1991
Buku Referensi