The Atlanta History Center is unsurprisingly the premier institution for Atlanta history. The Center is in Atlanta's Buckhead neighborhood, and has a small but focused library, and hosts both travelling & local exhibits.
The Atlanta-Fulton County Public Library is another excellent choice. The main branch, right downtown surpasses the History Center in its collection of old newspapers & magazines.
The Atlanta City Government has a page justifying the Landmark Status of The Ponce. It places the building in context in Atlanta's history.
The Atlanta City Government has a brief history of Atlanta on its website.
The Georgia Secretary of State gives access to online state historical archives, featuring contact information for local historical organizations, and photographic collections.
A good starting point is The New Georgia Encyclopedia's article on Atlanta.
Atlanta's history community remains an active one, with both amateurs & professionals putting together information in novel ways. One such site, recently opened is ATL History, with frequent posts on local lectures, history volumes & a forum for enthusiasts.
Roadside Georgia has an extensive history of Atlanta extending back to pre-historic times.
The Wikipedia page for Atlanta includes some historical information.