Sashkin, Marshall, 1944-
American Managemant Association, 1982
Buku Teks
edited by Kreg Lindberg and Donald E. Hawkins
Ecotourism Society, 1993
Buku Teks
Thompson, Leigh L.
Pearson - Prentice Hall, 2008
Buku Teks
Sweet, Donald H.
Addison-Wesley, 1975
Buku Teks
Stewart, Rosemary
Macmillan, 1993
Buku Teks