CRC press, 2020
eBooks
Hunt, Elgin F.
MacMillan, 1987
Buku Teks
Sumner, William Graham
New Haven Yale University Press, 1950
Buku Teks
Bierstedt, Robert
McGraw-Hill, 1964
Buku Teks
edited by Margaret Park Redfield
The University of Chicago Press , 1963
Buku Teks