Greenburg, Steve
Alpha, 2002
Buku Referensi
edited by Richard Mabey
Penguin Books, 1991
Buku Referensi
Collins, V.D.
J.B. Lippincott, 1935
Buku Teks SO
[research, writing, and illustrations, Charlie Wing ; editor Benjamin W. Allen]
Meredith Corporation, 1997
Buku Referensi
edited by Benjamin W. Allen
Better Homes and Gardens Books, 1997
Buku Teks SO