Superb, incredible site by Greg Germani showing before and after pictures from a huge collection of postcards and old photos collected from the Atlanta History Center Kenan Archives, GSU Pullen Archive and many other sources!
AtlantaTimeMachine.com

The Amazing Business Empire in Buildings of “Buddie” Candler (Asa Candler, Jr.)
https://w ww.asasbriarcliff.com/real-estate :

Old Newspaper Archive 17,600+ newspapers from the 1700s–2000s:  https://www.newspapers.com/

Maps Showing the Urban Growth of Atlanta: Very cool maps, one with an hour glass that when panned over present Atlanta shows 1949 map of same area.
https://gis.atlantaga.gov/storymaps-3-col.html

Open Street Maps: Interactive map of Atlantan Old Buildings.
http://greencracker.pythonanywhere.com/

Historic Atlanta, Preservation Though Community Action: Historic Atlanta helps communities save and thoughtfully reuse Atlanta’s historically significant places through advocacy, education, and technical assistance.
http://www.historicatlanta.org/

Atlanta Maps 1895-1997 from U.S. Geological Surveys: https://ngmdb.usgs.gov/topoview/viewer/#13/33.7600/-84.4065