City Directories are a very helpful source of information. Let's say you know your ancestor's name and the town where he was living. You can find out specific information about his address and his job (if one was recorded).
Let's take this as an example. My great-great grandfather was Louis Kafka. I know he lived in the Chicago area. I found a 1910 Lakeside Directory, City of Chicago. I foud this entry:
Kafka, Louis J barber h 2103 W Division
This shows me he was a barber and lived on Division Street in Chicago. Obviously things change from year to year. It is interesting to see if the ancestor moved or stayed in the same house. Did he keep the same job or not? I found a 1923 Chicago City Directory. This is the information I found from it.
Kafka, Louis J slsmn Utility Securities Co h1416 Bryn Mawr av
This shows me his job and where he lived. He was a salesman at Utility Securities Company and lived at 1416 Bryn Mawr Avenue in Chicago. (H stands for home.)
Most of the people listed in the directories are men; however, widows are listed as are women who had jobs. So, what did your ancestor do for a living and where did he live? Look them up. Directories are still available today, so track yourself and see where it is documented that you lived or worked. It might surprise you!