Stuart, Peter C.
Far horizon books, 1989
Buku Teks
Murdock, George Peter, author
The Free Press, 1965
Buku Teks
Kropotkin, Petr
Extending Horizons Books , 1955
Buku Teks
Geger Riyanto, author
LP3ES, 2009
Buku Teks
Hess, Beth B.
Macmillan Publishing Company, 1985
Buku Teks