Newbigin, Marion I.
Dent & sons, Ltd., 1925
Buku Teks
Marsh, William M., author
Cambridge University Press, 2013
Buku Teks
Strahler, Alan H., author
John Wiley & Sons, 1997
Buku Teks
Petersen, James F., author
Brooks/Cole Cengage Learning, 2012
Buku Teks
Whittow, John B., author
Allen Lane, 1984
Buku Referensi