Spear, Percival
The University of Michigan Press, 1961
Buku Teks
Brown, Joe David
Time Incorporated, 1961
Buku Teks SO
Hobsons Pub.,, 1987
Buku Referensi
Rybak, Stephanie
Macmillan, 1985
Buku Teks SO
Williams, Gareth
PSI, [date of publication not identified]
Buku Teks