Judaism 101
Judaism 101: Bar Mitzvah, Bat Mitzvah and Confirmation
This site does a nice job in explaining how the Bar & Bat Mitzvah have evolved. It also contains information on Jewish Confirmation, and includes many links to related topics.
American-Israeli Cooperative Enterprise
Jewish Virtual Library: The First American Bat Mitzvah
Enlightening story of the first Bat Mitzvah to take place in America in 1922.
Other
Bar & Bat Mitzvah
This site explains what the "right-of-passage" ceremony means to a Bar/Bat Mitzvah, including the historical aspect of how Abraham and his younger son are involved.