Wizards recently announced a new planar book due out later this year called "The Plane Below" or something. I'm guessing this is a crunchier planar book focused on a few places (or maybe just the Shadowfell). I sort of wish I had waited for this but I did use the fluff in a recent adventure describing how the party traveled from the Pyramid of Shadows floating in the Far Realm all the way through the Elemental Chaos, the Shadowfell, and finally home.
