Grolier academic encyclopedia 5
London: Grolier International, 1991,
 Buku Referensi
Grolier academic encyclopedia 7
London: Grolier International, 1991,
 Buku Referensi
Grolier academic encyclopedia 9
London: Grolier International, 1991,
 Buku Referensi
Grolier academic encyclopedia 10
London: Grolier International, 1991,
 Buku Referensi
Grolier academic encyclopedia 11
London: Grolier International, 1991,
 Buku Referensi