I started mine after reading Irilean in White Dwarf magazine back in about 1983. It's been revised and modified to make it adaptable to high and low magic but it's strength is the low-level whimsy, humour and flexibility, I think: https://www.drivethrurpg.com/browse/pub/14527/Dunromin-University-Press?affiliate_id=295776I'm in the middle of writing a city supplement for Shadowdark and it has occurred to me that there are a lot less in the way of design exemplars and best practices out there for city supplements. I don't want to restrict the conversation to just fantasy games either. It seems like city supplements are ripe for the kind of over-written lore dump that I cordially dislike in my RPG books, and a survey of my collection hasn't changed my mind much on that score.
So what are your favorite city sourcebooks, or specific mechanics, or procedures, or rules? You might even tell us why!

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.