Grolier academic encyclopedia 7
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
Grolier academic encyclopedia 13
London: Grolier International, 1991,
 Buku Referensi
Grolier academic encyclopedia 21
London: Grolier International, 1991,
 Buku Referensi