So you are interested in learning more about Planescape or even running/playing in a Planescape campaign. Where do you start? Well, presuming you want to dive in at a faster reading pace than this blog, I would most recommend purchasing a PDF of one of the original products from DMs Guild. Thankfully, the Planescape products were very heavy on the setting flavor and lore, without a great deal of mechanics that would need updating to 5e.
For the Player
As a player, the best resource back when Planescape was actively published is still the best resource now. The Planewalker’s Handbook (also available in print from DMs Guild). It is about 154 pages of content and is specifically designed to be everything a player would need to understand the Planescape setting.
Chapter 1: A Planewalker’s Guide to the Planes starts with a couple pages about the basic concepts of the setting before 22 pages explaining Sigil and the various planes. It isn’t a lot more information than what is in the 5e Dungeon Master’s Guide, but if either as a player, you don’t have the DMG or are interested in how the planes were portrayed when Planescape was published, this is a nice overview.
Chapter 2: Traveling the Multiverse gets into how portals work, which is independent of any game edition and the best ways to travel the planes.
Chapter 3: Getting There or Just Getting By: Planewalker Tips is some fun bits of advice for PCs in a Planescape setting, and even if it gets into particular spells to prepare, that section is brief and mostly discusses them conceptually in a way that can be useful even if they specifics of the range or casting time of a light spell changes through the editions.
Chapter 4: The Factions is an excellent 14 page overview of the faction basics. It is enough to give you a general idea, and from there if one strikes your fancy, you at least know what to look for elsewhere.
Chapter 5: Races and Archetypes has a decent amount of information about the races and only a minimal amount of mechanics information. It is also the first appearance of aasimar and rogue modrons as PCs, and first appearance of genasi ever, I believe. The archetypes referred to here are pure character concepts without mechanics. Like the mysterious “Berk without a Past” to “Planar Trader” merchant PCs to “Refugees from Darkness” fleeing the Lower Planes in hope of safety elsewhere. There are 16 altogether, and are rather similar to 5e backgrounds in that they can apply to any race or class and help shape the role-playing aspects of your character.
Chapter 6: Planar Kits and Proficiencies is mechanics heavy but is only 12 pages. Second edition AD&D kits are quite similar to 5e subclasses or Pathfinder archetypes if you are familiar with those. They are ways to customize a class to make it bit more specialized or have a slightly different flavor. These could serve as inspirations for 5e subclasses quite easily.
Chapter 7: Magic on the Planes explains the complexities of magic working differently on different planes. We will be looking more in depth into that in a Rule of 3 post coming very soon. It’s an interesting idea conceptually, but in practice, it can be very confusing and unwieldly. There’s also 20 pages of new spells that could be good idea mining, but of course aren’t usable as is.
Chapter 8: Planewalker Equipment is brief but full of ideas that can work across editions with very little tweaking from Baatorian Green Steel to equipment for Modrons.
Chapter 9: Planescape Campaigning is an excellent chapter for player and DM alike, and is packed densely with ideas on how to run Planescape campaigns, and potential campaign ideas.
Overall, although there are sections of outdated mechanics, the incredible thoroughness of explaining the basics of every single aspect of Planescape makes this still an excellent starting point a player. Other sources, of course, go into incredible depth on just about every topic in this book, but this is the book that covers everything, and will get even a 5e player the basic understanding of Planescape.
For Dungeon Masters
Dungeon Masters can get a lot of useful information from the Planewalker’s Handbook as well, but for them, the best place to start is the original Planescape Campaign Setting boxed set. It includes 4 books and some posters.
Player’s Guide to the Planes is what you get from the Planewalker’s Handbook but condensed down from almost 160 pages to only 32. If you have a player who is interested in Planescape but not so much as to read the Planewalker’s Handbook, this is the perfect place to start. The Faction information is even just slightly better than in the Planewalker’s Handbook (and taking up 15 of the 32 pages with a full page per Faction), and, for historical interest, is the first appearance of tieflings in D&D. (As well as bariaur, but they obviously haven’t caught on nearly as much.)
The DM’s Guide to the Planes is the next book, but the first few pages of introduction are good but unfortunately, after that you can largely ignore the rest for now. It goes into great detail about the varying effects of planes on spells and magic items, and then gives a general overview of the planes, but the 5e DMG information is sufficient.
Sigil and Beyond, however, is a 96 page guide to the City of Doors and the plane of the Outlands. Actually, first it begins with a 13 page “How to run a Planescape campaign” advice section that is great, and should have been in the previous book, but there’s that pesky thing about printing books in 16 page chunks. But it’s fine with us because you can largely skip that other book and just read this one cover to cover. The majority of it is then an in depth guide to the Outlands, including major towns and realms, and then a big section describing Sigil – what’s it’s like, how factions fit into the power structure, different city wards, etc. The Outlands and Sigil are great places to start a Planescape campaign setting because both the power levels are (usually) less lethal to lower level PCs and conceptually it’s a good place to start introducing ideas and themes before the Outer Planes crank them up to 11. This is a great setting guide with a ton of adventure potential that is the best place for DMs to start for running a Planescape campaign.
Finally, there’s a monster supplement that has 11 monsters plus all the ranks of modrons. So far only the lower 5 ranks of the modrons and 3 other monsters have been converted to 5e (as of this writing, post-Volo’s Guide and pre-Mordenkainen’s), and the monster write ups include a large amount of lore with the mechanics. The monsters range from Sigil-centric dabus to a mix of Lower Planar, Upper Planar, and Elemental creatures. The posters (divided up into separate sheets for the PDF) have a stylized mpa of the various planes, a more detailed map of the Outlands and Sigil, as well as a list of planar layers, deities by planes, and some artwork.
Overall, the Sigil and Beyond book alone is an excellent resource to starting a Planescape campaign with enough ideas to last possibly even last an entire 20 level campaign!