Spencer, Herbert
Watts, 1937
Buku Teks
edited by Frederick W. Moorman, revised by morris Palmer Tilley
DC Health, 1917
Buku Teks
Foreign Languages Publishing House, 1958
Buku Teks
American Red Cross, 1981
Buku Teks
Crystal, David, 1941-, author
Westview Press, 1980
Buku Referensi