Keeley, Joseph C.
McGraw-Hill Book Company, Inc. , 1950
Buku Teks
Schmookler, Jacob
Harvard University Press, 1972
Buku Teks
Petroski, Henry
Vintage Books , 1994
Buku Teks
National Geographic Book Service
National Geographic Society, 1988
Buku Referensi
Hickman, Bert G.
The Brooking Institution , 1965
Buku Teks