THE FACTOL'S MANIFESTO is one of my all time favorite Dungeons and Dragons supplements that, in simple terms, details the in's and out's of the fifteen factions of Sigil. Planescape is basically "Dungeons and Dragons + Philosophy 101" with factions representing atheism (Athar), self-actualization (Believers of the Source), Randism (Takers), Existentialism (Bleak Cabal), soliphism (Sign of One), Legalism (Fraternity of Order), Anarchism (Free League), and Fascism (Harmonium/Mercykillers).
It's all very goofy but when you're a teenager, it's fascinating to have the idea that these are BIG IDEAS being explored in what amounts to a city halfway between Ankh Moorpork and Good Omens. Actually, yes, Planescape had a VERY Pratchettian feel but didn't quite realize it was being humorous. Probably because Chris Avellone got ahold of it and made PLANESCAPE: TORMENT which treated its absolute ridiculouslessness with pure seriousness.
Either way, I loved this book and actually bought it in physical form again recently and gave a copy to a friend.
There is no setting like Planescape and this is one of the finest books for the setting. The books focuses on factions detailed after metaphysical pseudo-viable philosophy. The result is an interesting exploration of the most imaginative worldbuilding around. The book has everything from factions based on 2nd law of thermodynamics of entropy to benign nihilists. If you read it, disregard the mechanical implications and read it for the worldbuilding alone. There is plenty of inspiration to be found here.