A Guide to the Ethereal Plane (AD&D/Planescape), by Cordell, Bruce R.
Hellbound: The Blood War (AD&D/Planescape), by Monte Cook, Colin McComb
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
Planescape Campaign Setting (Advanced Dungeons & Dragons), by David "Zeb" Cook
Tales from the Infinite Staircase (AD&D/Planescape Adventure), by Cook, Monte