Another world exists beyond the one that humans can see - a world populated by demons. For centuries, demons have crossed into our world, some looking for a new life, adventure, or just a job. But many are intent on causing chaos and destruction, feasting on human flesh and fear. A tenuous peace treaty is the only barrier between the worlds, the only thing protecting humankind from utter devastation.

(Source: Macmillan).

Synopses for each volume can be found under more information.

Only the first novel was the basis for the Wicked City anime.