Greenwood, N. N.
Oxford Pergamon Press 1984,
Buku Referensi
Kice, John L., author
New York Macmillan Publishing 1974,
Buku Teks
Strahler, Alan A., author
John Wiley & Sons, 1996
Buku Teks
Elliot, Diane L
Philadelphia Lippincott - Raven 1997,
Buku Teks
Adamson, Arthur W.
New York John Wiley & Sons 1990,
Buku Teks