Armed with that data, one can search for naturalization records more easily. Not everyone was included in these old directories. Ancestors might not have been at home when the data was compiled or refused to answer the canvasser's questions. Moreover, in some cases, working class families are excluded, as are entire ethnic neighborhoods. Many directories are not alphabetized accurately, and there can be typographical errors and variant spellings of surnames.
City directories are widely available in libraries.
While most public libraries, historical societies and state archives have rather extensive collections of their own in-state directories, some have directories of other major cities. Many older city directories are available in microform and can be accessed through the Family History Library and its local branches throughout the country.