Cobb, Nancy J., author
Mayfield Publishing Company , 2001
Buku Teks
William Damon, editor-in-chief
John Wiley & Sons, 1998
Buku Referensi
Travers, John F.
Scott, Foresman, 1982
Buku Teks
Maier, Henry W
Harper and Row, 1978
Buku Teks
Gesell, Arnold, author
Harper & Row, 1949
Buku Teks