Lucas, P.H.C.
Chapman & Hall, 1992
Buku Teks
edited by Kreg Lindberg and Donald E. Hawkins
Ecotourism Society, 1993
Buku Teks
Lynton, Rolf P.
Carolina Population Center, University of North Carolina , 1974
Buku Teks
Walinsky, Louis J.
McGraw-Hill , 1963
Buku Teks
Hyman, Herbert H., author
An Aspen Publication ,
Buku Teks