A Guide to the Ethereal Plane (AD&D/Planescape), by Cordell, Bruce R.
In the Abyss (Advanced Dungeons & Dragons, 2nd Edition), by Williams, Skip
On Hallowed Ground (Advanced Dungeons & Dragons: Planescape, Deluxe Campaign Accessory/2623), by Colin McComb
Planes of Conflict (Advanced Dungeons & Dragons, 2nd Edition: Planescape, Campaign Expansion/2615), by Monte Cook, Bruce Nesmith, TSR Inc, Kevin Melka, Dale Donovan, Colin McComb
Planes of Law (AD&D 2nd Ed Fantasy Roleplaying, Planescape Campaign Expansion, 2607), by Wolfgang Baur, Colin McComb
Planescape Campaign Setting (Advanced Dungeons & Dragons), by David "Zeb" Cook