Reddit reviews Ptolus City by the Spire:

Try Monte Cook's Ptolus City By The Spire. The hardcopy is OOP and crazy pricey, but you can probably find a digital version somewhere. It's a massively detailed and cross-referenced city/campaign setting with a huge medieval-fantasy city situated next to a massively tall stone spire that used to be the home base of a long dead ancient BBG, and sitting atop a huge labyrinth of dungeons and cavern systems.

Amazon Link

OR, if you prefer random tables that help you improvise the city, check out Zak Smith's Vornheim: The Complete City Kit. Rather than detail every nook and cranny and joe blow under the sun, the thin book (also OOP but pdfs are plentiful and cheap) contains a series of random tables and quick techniques and die mechanics that allow you to either create a dark fantasy for your upcoming campaign/adventure, or generate everything from NPCs, streets and floor plans, chase sequences, shops, nobles, court hearings, etc, on the fly right in the middle of your game. So if the Rogue says "I duck into a side alley and through the nearest door, what's in here?", you can just (roll roll roll), it's a bakery, here's the layout (draws random floorplan), and so on

Vornheim link

Both are spectacular at what they set out to do, but just go about it in different ways.

Well - if you want to get super crazy - as in - I can set an entire campaign in this city and basically never have the players leave if they don't want to. Look no further than this:

http://www.amazon.com/books/dp/1588467899

This is a link to the hard copy - I'm sure you can find a PDF somewhere for cheaper. But yeah basically this book is the single best book I've ever read about a city. You could adapt it to any fantasy or D&D style setting basically and it has hundreds of NPCs, rumors, and dungeons pre-mapped out. In my own mind this is the gold standard of cities.

If you're looking for a city with a more Asian flair I can recommend http://www.rpgnow.com/product/3566/City-of-Lies-Box-Set?it=1&filters=0_0_10109_0_0. Though it is heavily steeped in the Legend of the Five Rings setting - it too is very detailed and has a bunch of stuff going on with it.

If you're looking for something more modern I can recommend the city building book for Vampire the Requiem http://www.amazon.com/Vampire-Damnation-City-Requiem/dp/1588462676 - I found this very helpful when I was playing world of darkness.

> would you buy a campaign setting sourcebook?

Yes. Have done so before, will do so again.

> If so, what features would you look for?
> If you've bought them in the past, then what made you want to buy them?

Maps! Descriptions of things on those maps. Interesting personalities. Story stuff.

When it gets to mechanical stuff I could live without them.

As a GM I can come up with stats that balance for my group. I can come up with cool toys. I can come up with fun tactical events.

I buy sourcebooks for the content which takes a lot of time to create: The maps, the personalities, the political conflicts, the broader strategic issues that are occurring.

Example: Ptolus. I have used and re-used people, places, and plots from Ptolus in numerous ways. One of my best RPG purchases ever.