God Bless America
The Library of Congress has posted a small site dedicated to the song. It includes some background information and a handful of documents.
There is a nice page dedicated to Irving Berlin at this site. It offers a solid biography and some interesting images and links. There is also a separate page dedicated to Tin Pan Alley.
The Songwriters Hall of Fame has compiled some useful materials for this online exhibit. The audio files are only teasers, but they give you a taste of Berlin’s music. There is also a separate exhibit for Tin Pan Alley.