Bray, Mark
The Commonwealth Secretariat, 1986
Buku Teks
Hammersley, Martyn, author
Paul Chapman Publishing, 2002
Buku Teks
Cleveland, Ohio, Lincoln Extension Institute
Buku Teks
Wiersma, William, author
Pearson/Allyn and Bacon, 2005
Buku Teks
Wiersma, William, author
Pearson, 2009
Buku Teks