Hi all, I am really interested in running a kind of planescape game and I am wondering what the best book is to set something like this up. I don’t particularly care about system specific mechanics, so feel free to list anything, this includes adjacent stuff like Monte Cook’s Planebreaker or
@SlyFlourish ‘s City of Arches! The most recent wotc book got mixed reviews, which of the old ones stand out?
If you're the sort of GM who can improvise mechanics and isn't looking for "devil in the details" but more of "vague and evocative to inspire your own adventures within the creative play space of Planescape", then I have two recommendations:
AD&D Planescape boxed set, and playing through
Planescape: Torment CRPG.
IME (ran Planescape for 5 years), those two products capture the feel and what makes Planescape unique more than any other products released for the line or adjacent products. I recommend the CRRG because I never felt like any of the adventures for Planescape (including the 5e one) really explored what makes the setting more than "D&D session on a weird plane." Whereas the CRPG does a
great job of exploring those unique themes in a way that's fun to play through. If you've got GM experience, I would use those two products as your lighthouse / governing principle/example of how to do Planescape.
Edit: Also, search for "planar renovation project" - this was a fan effort that started on Planewalker.com or maybe Mimir.net - which has some really vividly reimagined Outer Planes with the objective of making them more adventuresome in an actionable way for the GM (Rather than the PS books which tend to be very vague/abstract to the point of being difficult for some GMs to implement).
Edit Edit: Planar D&D and Planescape are distinct. Planar D&D is
A Paladin in Hell or other fun bizarre journeys to the City of Brass or what have you -- 3rd party products already mentioned do that very well, and Hellbound was a standout adventure from the PS line for "planar D&D".
Planescape is rooted in books like Perdido Street Station, Bangsian/afterlife fantasies, landscape and culture being directly influenced by belief, metaphysics, theories of multiple selves, the eternal return, and a lot of other heady topics.