Leighton, Angela
Cambridge University Press , 1984
Buku Teks
The Modern library, 1935
Buku Teks
Whiting, Percy H.
Dale Carnegie & Associates, Inc., 1979
Buku Teks
Ansell, Rod
Currency Press,, 1986
Buku Teks
Sykes, Percy Molesworth
Macmillan and co., limited, 1930
Buku Teks