Beyond Countless Doorways is a D20 supplement published by Monte Cook's Malhavoc Press in 2004. Believed by many to be the 3.x D&D book for Planescape-like games, after all, Monte Cook gathered Planescape alums Wolfgang Baur, Colin McComb and Ray Vallese together to write it.
There's one review on RPG.net (1).
Gamma Red Death World
8 months ago