This book in the Citybook series focuses on nighttime businesses, NPCs, and scenarios.
From the front cover:
A game-master's aid for all role-playing systems...18 nightside businesses and cultural establishments, and more than 60 fully developed non-player personalities with scenario suggestions for use with any role-playing system.
From the back cover:
If the streets of your city are growing too tame, slip across the shadowy borders to the little-known dark side—if you dare! For unexpected adventures, formidable foes, and dangerous predicaments, come to Deadly Nightside—it's the part of town where you're most likely to be caught dead!