Here is a quick look at something free that I like.
A Magical Medieval City Guide is a FREE PDF over on SJG's e23.
It's 40 pages of city-making goodness.
It does assume a bit of a magical economy, and so it fits most smoothly into a D&D-ish "I pop down to the magic store for some potions and a Continual Light spell" kind of game. But it's not so thoroughly so that it's useless for a less-freely-purchasable magic economy.
But in general, it's an excellent look at a European-style city with magic as an assumed reality. It gives you:
- a good explanation of guilds (and why "the mage down the street undercuts my prices" wouldn't be an issue)
- a good explanation of city rents to the lord, and the whole big city vs. peasants/serfs are tied to the land tension.
- how trade and pricing works
- city politics
- and lots of other little bits along those lines.
It's 40 pages, it's free, and it's easy to read and easy to reference. If you want to put together a city that feels pretty solid, this is a great reference point.