Shemeska said:More 2e Planescape stuff:
'Faces of Evil: The Fiends' and 'Hellbound: The Blood War'.
The first is the seminal sourcebook regarding the fiends of DnD, and the latter is the primary source of information regarding the Blood War, its history, and its impact across all of the planes. If you like fiends, these two books are among the must haves.
I also liked the 2e 'Secrets of the Lamp' by Wolfgang Baur, for the level of detail it give to the genie-kin of the inner planes.
Psion said:Book of the Planes has a bit more of a Planescape style, and would probably be better for you if you want to make organizations important in a game. The planes it provides are more in line with the traditional D&D cosmology.
Portals and Planes is a bit more off-beat, and entertains more different options for how you might run a planar game. One even inspired me to start a campaign (see my River of Worlds thread.)