Good "spirit world" material?

[LoneWolf23]

Surprised no one has mentioned the Ghostwalk campaign book.

That, and you could also try Oriental Adventures, which has a Shaman Class, a Spirit Plane replacing the Astral, and a Spirit monster subtype which can be overlayed to a lot of different monsters, making them into spirits. ...And not all of them being incorporeal creatures, either. And the book has spells dealing specifically with spirits.

Another good book to swipe from is Frostburn, which has the Spirit Animal template, which can be applied to any Animal to make them into spirit creatures.

 

[John Q. Mayhem]

Arcana Unearthed has some spirit-y goodness. A whole slew of classes based on animal totems, the greenbond who can see and communicate wuth nature spirits, and the witch class are good for such things.

 

[Psion]

I'm not sure if it's exactly what you want, but perhaps the Spirit plan in the 3E Manual of the Planes might be of some help?

That's a good start. I would also recommend GR's Shaman's Handbook, which has further bits on cosmology and creatures which fit with it nicely.

If you don't mind editing out some campaign specific stuff, there's some nice in-depth stuff in Rokugan's Fortunes and Winds.

Best of all, this is all d20 stuff, so you don't have to worry about translating stuff from other systems or prior editions.