City Infernal by Edward Lee

When Cassie's twin sister, Lissa, commits suicide, Cassie discovers she can travel to Hell to retrieve her sister's soul. Cassie thought she knew all about the Hell of legend, but finds Hell has evolved over the millennia into a bustling city full of the damned with looming skyscrapers, crowded streets, systemized evil, and atrocity as the status quo. Welcome to the Mephistopolis Hell is a city. It stretches, literally, without end-a labyrinth of smoke and waking nightmare. Just as endlessly, sewer grates belch flame from the sulphur fires that have raged beneath the streets for millennia. Clock towers spire in every district, by public law, but their faces have no hands; time is not measured here in seconds or hours but in atrocity and despair. In the center of this morass of stone and smoke and butchery and horror stands the 666-floor Mephisto Building, where Gargoyles prowl the wind-blown ledges and from whose highest garrets the innocent are hung from gibbets and left to rot for eons. The lone occupant of the very top floor looks down upon his dominion and smiles a smile that is brighter than a thousand suns. Here, yes, everyone is dead yet everyone lives forever. Welcome to the Mephistopolis. Welcome to the city of Hell. Welcome.

This book is part of the "City Infernal" series.
Here are some other books from this series: