Spear, Percival
The University of Michigan Press, 1961
Buku Teks
Rybak, Stephanie
Macmillan, 1985
Buku Teks
Williams, Gareth
PSI, [date of publication not identified]
Buku Teks
Sydsæter, Knut
Prentice-Hall, 2008
Buku Teks
Hobsons Pub.,, 1987
Buku Referensi