Taylor, James
American Management Association, 2006
eBooks
Thompson, Leigh L.
Pearson/Prentice Hall, 2004
Buku Teks
Sweet, Donald H.
Addison-Wesley, 1975
Buku Teks
edited by Kreg Lindberg and Donald E. Hawkins
Ecotourism Society, 1993
Buku Teks
Stewart, Rosemary
Macmillan, 1993
Buku Teks