Library of Congress on "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.
Parlor Songs
There's a nice page dedicated to Irving Berlin at this site. It offers a solid biography and some interesting images and links. There's also a separate page dedicated to Tin Pan Alley.
Songwriters Hall of Fame
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's also a separate exhibit for Tin Pan Alley.