Despite the "d20 glut", the number of city settings one can choose from can be counted on one hand. This is a pity, as the city offers NPC interaction, combatless skullduggery, and other entertaining roleplaying not available in your everyday dungeon or wilderness adventure.
Enter "The Kaiin Player's Guide". Technically for the Dying Earth RPG, based on the works of Jack Vance, the KPG features one of the most entertaining fantasy cities I've read in a long time. Written by Robin D. Laws, designer of Feng Shui, Hero Wars, Rune, and aforementioned Dying Earth, the book is 200-some pages long, on heavy glossy paper. It's quite suitable for D&D or any other fantasy roleplaying game.
For GMs of Conventional Fantasy RPGs
Let me briefly mention what the book doesn't have. The maps have a "bird's eye view" of each section of the city, but not detailed maps of streets or locations. Stats for NPCs are for Dying Earth's skills, which focus on personality, and thus are quite usable in any roleplaying game. You'll definitely want to plug in the culture, personalities, and locations of Kaiin to a city with detailed street-by-street maps.
The information is portrayed from the perspective of a well-connected PC. As a result, the GM need no longer be a bottleneck for information. He can let the player peruse the book, or a particular section. Not all GMs will want to do this, but now they have this option. GMs will need to create any "secret" character details and specifics (not like they haven't had to do this with other city guides...)
For Dying Earth Players
As said, this book is written from the perspective of a resident of Kaiin. Despite the words "Player's Guide", there is no "Gamemaster Guide". The player peruses the book, selects a rumor, and the GM improvises, using the GMC creation rules. A few pages provide additional advice for the GM and player.
A City of Personalities
Most city books I've read are organized by locations. Each chapter discusses one of the quarters by providing an overview, then particular locations, each with their NPC residents (don't these people ever leave the house?). KPG provides an overview, but then focuses on the PC's contacts for the city and personages in the quarter. Contacts are a personality outline of a particular class of people in the quarter, which the GM (or player!) fleshes out as an NPC contact. Personally, I think contacts are a much more playable NPCs than the usual "Important People" section of a city setting.
Locations, calendar events, and rumors follow, then Taglines and Tweaks. Taglines and Tweaks are Dying Earth mechanics. Taglines are quotes which, if used by a player during play ("Call me parsimonious, but I am reluctant to become nourishment for anyone."), grants character improvement points. Tweaks are special bonuses that a character can have under certain social circumstances for their PC. Both should be adaptable to any fantasy rpg. Kaiin is composed of neighborhoods: Canal Town, where the city river "emits its last pathetic gasp before it empties into the bay" (avoid the oysters); The Fringe, home of the most destitute denizens and opportunistic bandits (razed to the ground periodically with lively festivals involving hangings); The Marketplace (including no less than three pages of cart-pulling "Unusual Mounts and Beasts"); Odkin Prospect, where reside the aristocrats and merchants they invest in and despise; The Palace & Environs, including the useful "Palace Intrigue Quick Reference" chart; The Scholasticarium District, an institution less of learning than of "rivals in an never-ending struggle for privelege and prestige"; The Threek, a medieval equivalent of the suburbs ("Although eighty percent of the population lives here, we cover it quickly"); The Tracks, where Dhejtar, a sort of cross between a weasel and a panther, race (or just bring your child here to throw rocks at defaulters in the punishment racks); and The Undercity, who has believed "the sun has already died, and they are the last survivors" (enjoy the spicy cuisine).
Again, I'll emphasize that this is a book of personalities over locations. GMs looking for detailed maps of a city won't find them here, but should still consider KPG as a book of NPCs who will enliven any city. GMs who do not improvise city adventures will still need to create and prepare material. Still, the wealth of NPCs, especially the contacts, should enrich any city adventure, not just one for Dying Earth.