Directories have been published from the eighteenth century, listing those in business and notable residents. It was not until the nineteenth century that a higher number began to appear however.
They listed tradesmen, craftsmen, merchants, professionals, farmers, clergy, gentry and nobility, but as time passed, they also began to include private residents, whether they were rich or poor.
From the nineteenth century they were often divided into Commercial, Trades, Streets and Court sections. The Commercial section covered professionals, farmers and traders, entered alphabetically.
The Trades section was a list of the foregoing, but this time listed by trade or profession. The Streets section was a list of private residents and tradesmen, listed street by street and house by house.
The Court section started off being a schedule of heads of wealthier households, but became a list of the heads of most families, excluding the poor.
It is possible you could discover an ancestor previously unknown to you. If you find other people with the same surname as your ancestor in the area they lived in they may be their relatives, especially if their surname is less common.
If the year was close to a census year, looking up the address in the census return could lead to you discovering more family members, because census returns gave more information such as age and place of birth.
A book from 1885 could in fact cover the year of 1884. The books did not give much information about individuals, so if your ancestor was named Philip Williams and was a farmer, it is possible an entry could in fact relate to his son who had carried on the family business as Philip Williams, senior, may have died or had retired.
Telephone Books began to appear during the 1880's, taking over from Street and Trade Books after the Second World War. Coverage of telephone books is incomplete because not all households had telephones.
They only usually list the main householder and not other household members, but they do list the person's address and exchange up to 1968. Older books may include adverts for local businesses, which may be one your ancestor was connected with. Telephone books were usually published every one or two years.
I have found a wonderful online directory that lists many family history resources.
Another site that lists online resources is Genealogy Links. Although their site concentrates on Cambridgeshire, Derbyshire, Lincolnshire, Nottinghamshire and Suffolk, they also have links to other national websites you may find of use.