Thompson, Leigh L., author
Pearson/Prentice Hall, 2004
Buku Teks
Sweet, Donald H., author
Addison-Wesley, 1975
Buku Teks
edited by Kreg Lindberg and Donald E. Hawkins
Ecotourism Society, 1993
Buku Teks
Stewart, Rosemary, author
Macmillan, 1993
Buku Teks
Formisano, Roger A., author
McGraw-Hill, 2004
Buku Teks