Green Lantern: Mosaic

The mad Guardian Appa Ali Apsa, driven insane by loneliness, uprooted cities from across the universe, including Evergreen City on Earth, and set them down on Oa. When the Guardians return, they decide to let the experiment play out, and they put John Stewart in charge of the Mosaic World.

Mosaic is a quieter, more thoughtful Green Lantern book. John has to keep the peace between communities that never chose to live together and try to build one society out of them. It's one of John's defining stories, and ends with him becoming the first mortal Guardian.