3e Manual of the Planes was very, very good. It's probably my favorite 3e book thus printed. The Planar Handbook was useful as well.
That said, all the 3e resources on the planes are really just watered down Planescape. If you want more comprehensive details you'll want to check out the 2e resources (Planes of Chaos/Conflict/Law, 'Hellbound: The Blood War', Guide to the Astral, Guide to the Ethereal, Inner Planes, 'Faces of Evil: The Fiends' etc).
They are permeated with an atmosphere of grandeur, scope, and capacity for wonder and mystery that I personally find lacking in material before or since. A lot of the complaints by some folks about the PS 'feel' I find to be simply from lack of exposure to it, and that it changed the planes from big outer planar dungeons to living, dynamic places filled with that sense of infinite wonder.
I delve into my exploration of them in my storyhour, largely focused on the lower planes, which you might enjoy if you're thinking of running a planar campaign
