Vast is Vornheim, the Grey Maze
Vornheim: The Complete City Kit, says author Zak Smith, "is not about Vornheim. It's about running Vornheim, or any other city, in a fantastic Medieval setting." This focus on providing content that's useful for gaming, rather than a sociology textbook on a fictional setting, sets Vornheim apart from any other city supplement I've purchased.
That said, Vornheim does not leave fans of Zak's campaigns (well-known from his blog and video show) without the details they crave. Maps and descriptions for major landmarks such as the House of the Medusa, the Immortal Zoo of Ping Feng, the Library of Zorlac, the Palace Massive, and the Eminent Cathedral are all provided, as is a regional gazetteer of the area surrounding Vornheim and a five-page overview of the city. The real meat of the kit, however, lies in its rules and tables.
Here you'll find rules for city navigation and urban "dungeon crawls," geomorphs for rapidly generating floorplans, rules for pricing items, rules for libraries, rules for urban chases, and tables to randomly generate aristocrats, books, shopkeepers, encounters, fortunes, and more. Among the best are the "I Search the Body" table, which holds treasures such as "a satirical deck of cards depicting political figures of 10 years ago," and the Legal Situations table, with outrageously medieval punishments such as the "trial by biography" in which the defendant must recite his life story to the jury, and if they judge him to be a good and useful person, he goes free. There's also rules for "God's Chess" (a method of using ordinary chess to represent strategic city maneuvering), a variety of taverns and tavern games, several random building generators, and more. I have to date only enjoyed the PDF, as the print version is only available on pre-order, but the print version is definitely a must-have. Many of the components have obvious physical utility, such as a map labeled with building names that lets you just drop dice on the map to determine which buildings are present in any neighborhood.
In addition to being crammed full of useful game mechanics for urban play, Vornheim is also crammed with great explanations of the philosophy of its mechanics. For instance, most settings will urge the GM to fill in the gaps; in Vornheim, Zak explains why: "Any detail of the rules or setting left unexplained has been left that way because it's not important to the character of the setting, and the GM should interpret it however s/he feels will distribute maximum fun." The book is almost as useful as a zen primer for improvisational Gamemastering.
If you are running an RPG campaign that has any cities at all, Vornheim has a lot to offer - you can begin using it in your campaign right away; and as a student of Gamemastering you can study its construction to learn how to create flavor in the game rather than the meta-game. (Oh and the art's pretty good, too. They got some fancy fine artist to do it who exhibits in galleries and museums).