The new Book of the Righteous includes all the missing archons, guardinals, and eladrins, although shiere eladrins have been replaced with an entirely new kind of eladrin called shiradi.
Differences:
The tome archons now have individual appearances and descriptions, and they more explicitly parallel the Lords of the Nine. None of them have bird heads.
Owl archons are a new caste; though they look pretty much like noctrals they have no sage abilities. It's possible that they're young noctrals, still in training.
Sword archons now have arms, and the ability to transform them into fiery swords.
Throne archons have a very interesting power called the Penitentiary Gaze which affects characters differently depending on how chaotic or evil they are. They've lost some of their other spell-like abilities, and their swords are somewhat less powerful, but they fill the same role.
A new kind of guardinal, the musteval, is a race of ferret-like spies. Talisid and all five of his Companions are fully described and statted.
Morwel and her two consorts (she's explicitly bisexual) are fully described and statted.
Sunflies appear as swarm creatures, along with swarms of magic frogs, locusts, and ravens that can be sent as divine plagues.
Hollyphants can now transform into a mammoth-sized winged mammoth, as well as their traditional appearance.
Leskylors are winged cats from Elysium, I guess so cat lovers don't feel left out because of all the canine entities there (moon dogs and lupinals).
Rheks are explicitly tied to the Harmonium, which outside the celestials from Planes of Law and the 2nd PS monstrous appendix are the only explicit Planescape reference. This book seems to contradict the Manual of the Planes' assertion that the loss of Arcadia's third layer was caused by the formians thousands of years ago.
They do a pretty good job with the asuras, though their status as fallen archons isn't mentioned. They don't go overboard on the eladrins' elfiness, which is good -- the shiradi are hardly elfy at all.
Bariaurs are Large now. The book gives them a level adjustment of +2, though the bariaur class in the last issue of Dragon had five levels.
The only 2nd edition celestials that aren't in 3rd edition now are the buseni, balaena, noveri, agathinon, light aasimon, and good incarnates. Oh, plus opinici and foo creatures, I guess. And baku. Okay, there are still a lot to go.
The Deathless are a new creature type, the positive-energy equivalent of undead. They're souls that are enabled to interact with the living world through positive energy. The unborn soul things from the Book of Hallowed Might should be revised to be considered Deathless.
An error: The Book of Exalted Might claims lillends are native to Arborea instead of Ysgard. Well, maybe it's not an error -- it makes sense if you ignore the Gates of the Moon and the Infinite Staircase.
Each celestial ruler (they're usually called paragons or exemplars) has a proxy statted for us. Many of them have associated prestige classes, although they supposedly aren't worshipped.
They say Pistis Sophia "wants for nothing" when what they mean is that she desires nothing. I wonder if that was the author's fault or the editor's.
A lot of what's in this book is the opposite of the Book of Vile Darkness. Instead of vile feats we have exalted feats. Instead of Dark Speech we have the Words of Creation. Instead of human sacrifice we have self-sacrifice. Instead of evil poisons and diseases we have good poisons and diseases, or at least non-evil.
There's a feat where you can get a coure as a familiar.
I really like the idea of redeemed evil items.
The new gods of good are nothing special, but they do the job. There aren't any as interesting as some of the evil gods in the Book of Vile Darkness. There's a "mature audiences only" goddess.
There's a slight contradiction in the description of the sanctified template -- you aren't supposed to allow evil outsiders to be sanctified, but there are rules for doing so anyway. It would make a decent "risen" fiend template, but it's pretty weird for its intended purpose: giving celestial qualities to reformed mortal creatures of evil. Why? I liked the similar, but more varied, templates in Green Ronin's Avatar's Handbook better.
I like Baxa's art, but he may have been the wrong choice for drawing angels. The deva Evansheer is particularly unangelic looking.
The illustration of the half-orc paladin having to choose between slaying two evil succubi or honoring their true and noble love for one another has gained some notoriety, but it really has nothing to do with the rest of the book, which generally dismisses the possibility of fiends ever finding any form of redemption. This is a shame, in my opinion. The illustration is probably just something the editors inserted. Note that all the art directors and editors are female. For what it's worth.
There are some particularly fascinating celestial minerals, like Moonblood, Storm Tears, Frystalline and Ysgardian Heartwire.
The book overuses unicorns and half-orcs. It seems like every exalted character is a half-orc riding a unicorn. This impression is false, but that's how it seems to me.