Thanks to the efforts of a couple good friends, I'm actually putting out a system-agnostic setting book as part of ZineQuest 2:
This is just me and two friends from my regular game group, and I think the fact that we have been gaming in this setting is reflected in the art and design. One friend drew the map (in a style inspired by Dyson Logos). Another friend is doing all the internal art in a style designed to evoke period-appropriate woodcuts. I'm writing the words in style that I can only hope captures something of the serious unseriousness of the place (it is a lived in RPG setting, after all, and bears the quirky marks of actual play with pride) without, I hope, being too indulgent.
Here is a sample of the spread for "the Tower Cliffs," the neighborhood where most wizards make their homes.
This setting book is system agnostic, but it has been shaped and formed by the systems I've used to run it.
I first conceived of the setting for a Big Eyes, Small Mouth (2e) game that never happened. I'm not sure what influences that may have had. Possibly none, certainly none I can remember, but influences can sneak into things in forgotten ways. It first came to life through a Rules Cyclopedia D&D game centered almost entirely on heists of wizards' towers. The events of that campaign are indirectly chronicled in the "Personalities of the Rainy City" posts on this very blog. A couple years later, I ran a campaign of it using The Fantasy Trip, and I think TFT is the game system that most shaped the city you'll find in the Visitor's Guide. The book could be used directly with The Fantasy Trip with ease. Lately, I've been using my own homebrew system for our current campaign -- this system being an overcomplicated riff on Risus. I've also run a couple sessions in the setting using Castle Falkenstein and Warhammer Fantasy Roleplay (2e), and one pickup game at a local junior high school D&D club using D&D 5e. I have inchoate thoughts about using different game systems to run a setting and how doing so enriches the place, revealing things about it that might never had been revealed if you stuck to just one system, but I'll save those for another time.
In any case, there it is. I'm writing a system-agnostic setting book for a lived-in RPG city where it always rains. The Kickstarter launched today, and it looks like it has just funded while I was writing this post. Take a look, if you're intrigued.