Here you'll find an ever-growing list of resources that we used to create Dead Soldiers and, more importantly, that you might find useful for RP here.

General 1920s
1920s Slang
The Prohibition
Popular 1920s male names
Popular 1920s female names

Chicago
Encyclopedia of Chicago
Photographs from the Chicago Daily Nws, 1902-1933

General Crime in 1920s Chicago
Organized Crime in Chicago
Timeline of Organized Crime
Homicide in Chicago: Prohibition and Temperance
Prohibition of Alcohol in the 1920s
Genesis of Organized Crime in Chicago

Italian Mafia/Crime
Chicago's Unione Siciliana: 1920, A Decade of Slaughter

Irish Mafia/Crime
The Irish Mob

As stated, this list is ever-growing. If you run across any websites/books/whatever that you think would be useful, let us know and we'll add it to the list!