City directories can help locate an ancestor and you can often squeeze out quite a bit of inforamation. The Board of Certification of Genealogists has a sill building page "Analyzing City Directories" that gives some good suggestions for using them effectively. Other articles include:

Along those lines: city directories by George Morgan.

Sample pages:

1928 Butte Montana * typical city directory page * abbreviations explained * find the widows * Did your ancestor run a business? When did Anthony Koscielniak die ? name of wife is listed in parentheses * example of a map in a city directory * 1837 Detroit * Ann Arbor, Ypsi 1917 *

Availability

The place most likely to have city directories for a town is the public library of that town. Often state libraries and larger libraries will also have city directories for towns in that state. The Bentley Library has many city directories for Michigan cities and towns.

There is a major microform set called "City Directories of the United States", which is marketed to libraries and many libraries have portions of this set--usually, of course, covering the area that library serves. The Allen County Library in Fort Wayne has a full set I believe -- if not a full set, they have a significant portion of it. The Library of Congress has this set and a page that lists holdings.

Some City Directories have been digitized on Google Books.

Some city directories are on subscription databases Ancestry.com and genealogylibrary.com. There are a few searchable directories on Old Directories.com . This site also offers a list of some other city directories available online.

You can purchcase CDs of some old city directories.

There is a city directories mailing list for those who want to requeste lookups and are willing to do look-ups for this area.

Genealogy Research Associates offers an extremely helpful site, that includes lists to available directories for various states, research guidance and a bibliography of resources: City Directories of the United States of America.