I'm pretty psyched for the statblocks and reskinning/reflavoring content coming in Planescape 5e, but I'm disappointed that there won't be more in-depth info on the planes themselves, especially the upper/lower/law/chaos planes. I definitely want to write a plane-hopping campaign, and I'm sure with enough googling I could find what I need, but are there any resources that y'all could point out (fandom wikis, 5e or older edition books, etc.) that are particularly good?

Manual of the Planes is from an earlier edition and has tons of lore. I’d probably start there. 

So, it helps to know that the original Manual of the Planes by Jeff Grubb was the first really dedicated effort, coming out for 1e late in its phase. It was what inspired the development team led by Cook to create Sigil and start up Planescape after they had success with the 1e book and the Spelljammer stuff.

2e never got its own version of the MotP,  but they did release later versions for both 3rd and 4th. So it is the first major place. The version I would most strongly suggest is the 4th Ed version, because they used it to create and inform this version of Planescape.

Baldur's Gate, Descent into Avernus, Out of the Abyss, The Wild beyond the Witchlight, -- all good adventures to use to get a better idea of at least some of the places.

Spelljammer, of course -- it doesn't deal *directly* with the planes, but has an impact on the whole.

And the DMG. It gives some basic information on the major planes itself.

Quote from AEDorsay >>

So, it helps to know that the original Manual of the Planes by Jeff Grubb was the first really dedicated effort, coming out for 1e late in its phase. It was what inspired the development team led by Cook to create Sigil and start up Planescape after they had success with the 1e book and the Spelljammer stuff.

2e never got its own version of the MotP,  but they did release later versions for both 3rd and 4th. So it is the first major place. The version I would most strongly suggest is the 4th Ed version, because they used it to create and inform this version of Planescape.

Baldur's Gate, Descent into Avernus, Out of the Abyss, The Wild beyond the Witchlight, -- all good adventures to use to get a better idea of at least some of the places.

Spelljammer, of course -- it doesn't deal *directly* with the planes, but has an impact on the whole.

And the DMG. It gives some basic information on the major planes itself.

 

 

 thanks for this! what an incredibly detailed response, i feel the need to compliment you on your knowledge ghjfdsghd

As official 5E published settings go, Journeys Through The Radiant Citadel presents an alternative setting that I think meshes quite well with Planescape's more gritty European gutterpunk feel.

The Wikipedia entry for Planescape is an excellent primer as well. Not only does it go over the publication history, it has a simple overview of things. It's not as useful as buying one of the books or Planescape products, but it's informative and a good start.

Quote from AEDorsay >>

The version I would most strongly suggest is the 4th Ed version, because they used it to create and inform this version of Planescape.

 Interesting, given that 4e didn't use the Great Wheel.