For the purposes of filling out coverage of the Planescape line, I’ll just briefly cover here the various monster-oriented supplements put out for the line. Most 2nd edition settings got a bunch of these, but those for Planescape might have had the biggest job of any outside Ravenloft. Whilst Ravenloft‘s monster material needed to create creatures which could be viable gothic adversaries, rather than mere “slay me and take my treasure” gribblies, Planescape needed to provide suites of monsters to suit the schticks of each of the dozens of planes involved in the game.
The first Planescape Monstrous Compendium Appendix was largely there to do two jobs: replace the long out of print Outer Planes Monstrous Compendium Appendix, and put all the devils and demons (and modrons and slaad!) back into AD&D which the more controversy-averse approach by TSR management had caused to be left on the shelf in preparing 2E. It’s pretty solid, not least because a lot of the creatures outlined here are tried and true iconic entities from 1E. The second appendix concentrates on filling out the Outer Planes roster, including a stab at detailing the Rilmani, who are to the true Neutral Outlands what celestial are to the good planes or fiends to the evil planes or modrons to Mechanus or Slaad to limbo. This was an easy enough job to do because ultimately the Outer Planes are the fun, characterful ones.
The third Planescape Monstrous Compendium Appendix was put together by Monte Cook, and my heart goes out to the guy because he had the thankless task of trying to make the monsters of the Inner Planes (plus some Astral and Ethereal critters) interesting. (To that extent, it parallels the long slog he had to do on the official Inner Planes supplement.) To be fair, with the vast numbers of paraelemental and quasielemental planes cluttering things up, he had a better pallete to work with than just “fire stuff”, “water stuff”, “air stuff” and “earth stuff”, and he makes some headway with the idea of “Parallelism” (the concept that anything on one elemental plane will tend to have its equivalents on the other), and most of the individual monster entries are decent, but it’s a bit of a samey concept for a supplement and I suspect few Planescape games spend enough time in the Inner Planes to use most of these anyway.
Another monster-focused book from the line is Colin McComb’s Faces of Evil: the Fiends. This is a sort of extended meditation on the organisation and social structure of the fiends, presented as a collection of in-character essays (and, annoyingly, assumes a particular outcome to one of the adventures in Hellbound: the Blood War as canon – fuck off, metaplots, nobody likes you). It isn’t quite a Van Richten’s Guide to Fiends – notably because the Ravenloft line already did one – but it’s very helpful for fleshing out the internal structure of the infernal realms and could therefore also be very useful for any non-Planescape campaign in which the machinations of demons and devils and their cousins are significant elements.