A City Full of Sinners is the game master's guide for The Vigilante Hack. It's roughly the same size as the core book. While you don't need it to run a game using the hack, it's a damn fine book that's really useful.
The book starts with a discussion of city building and dice drops. This is followed by a brief description of Dreamsend, the worst neighborhood in the corrupt Apollo City. The book then presents a drop table and several pages of tables. These let you decide what buildings are in the neighborhood, then give you unique aspects of the buildings, denizens, and activities. There's also a handy little worksheet to go along with it. This is followed by an instant street gang drop table and a useful random villain generator.
The book has things for the players too. There are some nice kits that make vigilante work a lot easier. There are also rules for utility belts and additional equipment. The most important thing on this list is boomerang, because now I can alter the Archer class and make a heroic Captain Boomerang (that has trick boomerangs instead of arrows). There are also some gadget rules, which makes sense because many non-powered vigilante aren't much without their toys. These equipment rules are all optional. This is followed by vehicle rules, including things you can use to pimp your ride. The Vigilante Hack is a dark game and there's discussion of drugs, as well as some super drugs (primarily named after comic book heroes). This includes simple addiction rules. Since vigilantes are street level there's a page discussing contacts, which is a wonderful addition.
Finally there's a one page adventure for 1st level characters which is followed by a 3rd level Martial Artist that can bail the party out if they get in over their heads.
If you play The Vigilante Hack, you definitely need to pick this up. Actually if you play any gritty modern games, I think this is a useful book to have. You can grab A City Full of Sinners here.