Directories were published annually from the mid-nineteenth to the mid-twentieth centuries. The city directory functioned as an earlier, comprehensive version of the telephone book of our time. Beyond providing an alphabetical listing of names, city directories typically noted an individual's occupation, work place, or business, and sometimes religion, race, and gender.
Read the whole thing. You can also click on the link to Shirley's biography and see the list of articles she has written. Shirley is the 2006 President of the Genealogical Speakers Guild.
City directories have been extremely helpful to me in my research of my San Diego ancestry, and in answering queries from other researchers about their San Diego relatives. For families in mid-size to large cities, they can be indispensable, especially for the years between 1880 and 1900, and the years after the available census records.