I always love sources that can provide me with as much information as possible to make a campaign. I came across this map (split in two parts) which provides me with more than enough districts to build a working city. It shows middle and working classes by ethnicity which really helps define areas where mobs can be located. I have still to get myself a copy of the RPG, but I'm hoping that I won't have to wait forever.
Ward Map of Lakefront City (1920-1940)
Middle and working class German
City Center Mixed Ethnic and Class Groups
Upper Class Assimilated
Middle Class Assimilated
Working Class Assimilated
Working Class Irish
Working Class Italian
Jewish Lower Class (same nationalities as East European Working Class)
East European Working Class (includes Poles, Russians, Slavs, Croats, Czechs, Latvians, Lithuanians, Estonians, Hungarians and Rumanians)
Now I've dug out my copy of Family Business, time for some mob wars!