Boyd, Anne Morris
H.W. Wilson, 1949
Buku Teks SO
Walker, Edward Everett, 1900-1974
Charles Scribner's Sons, 1943
Buku Teks SO
Zink, Harold
Macmillan, 1948
Buku Teks
The America University, [Date of publication not identified]
Buku Teks
Holloway, William Vernon
McGraw-Hill , 1951
Buku Teks