I have recently been studying D&D Cosmology and was impressed by the level of detail in the descriptions of Baator and The Abyss found in Fiendish Codex I & II.
Are there any source books that explore the good aligned outer planes to the same detail? Specifically I want to learn more about Celestia, Elysium or Arborea.
No, there is not. The Upper Planes have always gotten short shrift in D&D, and 3.5e is no exception. Since most campaigns feature Good, mortal protagonists against Evil antagonists, there isn’t a lot of call for details on Good immortals and their residences. Fiend Folio and Fiendish Codex are marketed to DMs who want to use the fiends as foes in a campaign, which is a lot of DMs because most DMs have a Good party and fiends are the ultimate Evil to put in front of them. A book on the flip-side wouldn’t have nearly as large an audience, because evil campaigns are unusual and DM-focused books already sell relatively less-well.
As a result, the only real source we have on the Upper Planes is the Book of Exalted Deeds, which has material for both players and DMs. But even there, the details on the Upper Planes are limited to a few pages each; the bulk of the book is devoted to what it means to be a Good or better-than-Good player character, and what it means to DM for such characters.
In short, campaigns go into Hell or the Abyss and do things there. When campaigns go to the heavens, it’s usually because the campaign is ending and the characters are being rewarded. At best, it’s a quick stop to confer with some force for Good, a deity perhaps. The call for the details just isn’t there.
