Callow, Duncan
Chischester, 1995
Buku Teks
Callow, Duncan
Otter Publications , 1995
Buku Teks
Carvan, J.C.
LBC Information Service, 1997
Buku Teks
Barnes, A. James
Homewood, Ill., Learning Systems
Buku Teks
Twomey, David P.
South-Western Publishing Company, 1986
Buku Teks