Manual of the Planes is still my go to reference when creating planar adventures. Using it heavily now in my homebrew Sigil campaign.If we're lucky, maybe WoTC will update 3e's Manual of the Planes to 5e. I liked how that book mentioned what kind of gravity each plane had (objective vs subjective) and how each plane affected the nature of certain spells (heightening spells with one kind of descriptor while diminishing spells with another kind of descriptor). The latter in 5e will probably involve casting spells with Advantage or Disadvantage.
And while Alignment is not a big thing as it was in previous editions, Alignment did play a big part in the Outer Planes as well. For instance, the third layer of Arcadia was lost to Mechanus when the Harmonium tried to force Chaotic beings into being Lawful in nature. As a result, there was an alignment shift and an entire layer slipped away. Oops!