Backman, Jules
Brookings Institution, 1943
Buku Teks
Hamilton, Walton
McGraw-Hill Book Company , 1938
Buku Teks
Nourse, Edwin G
Brookings Institution, 1938
Buku Teks
George Allen and Unwin , 1951
Buku Teks
Dorfman, Robert
Prentice-Hall of India, 1970
Buku Teks