Spencer, Donald D.
Ginn-Blaisdell, 1968
Buku Referensi
Spencer, Donald D.
Camelot Publishing Company, 1993
Buku Referensi
Hill, Francis S., author
Macmillan, 1990
Buku Teks
Hearn, Donald, author
Prentice-Hall, 1986
Buku Teks
by Donald Greenberg [and others]
Addison-Wesley, 1982
Buku Teks