Free Rockford City Directories are available at HathiTrust for Rockford, Winnebago County, Illinois.
City directories contain an alphabetized list of residents with their addresses, spouse’s name, profession, and often a reverse lookup by address. They often contain historical information about a locale and they’ll usually contain a separate business directory.
HathiTrust currently has these Rockford City Directories available for free searches:
1923-1978 Rockford City Directories, limited to search only, no images