Several City and Business Directories have been uploaded to the Internet Archives website and to the Digital NC Collections. A new Directories page organized by year has been added to the Durham County site here.
City directories can be very useful tools for genealogical research! They are alphabetically arranged making them easy to use; they are usually compiled each year; and they take care to capture all households of the city.
Because these directories were created more for business use, the creators took care to be exact with name spellings, middle initials, addresses, etc because they needed to be able to distinguish between their customers.
You can read more about why directories were created and how they can be used in genealogical research in this article by the ProGenealogists.