So long as the book or books didn't take pointers on morality from the BoED.
That said, there's plenty of ground to cover. The Eladrin and Guardinals could use more detailed material on their society and history, and an exploration of their goals in the multiverse at large. The celestials as a whole should be presented as a complex, sometimes fractured group, not a monolithic Good, but at the same time the different way they handle their own divide along the Chaos/Law axis versus how the fiends do so is rather stark and important. Now how the Archons handle that w/ respect to Asuras, that's another topic that's full of fun.
And having just mentioned them, I hate to say it but the Archons have been done to death, because of all the celestials they tend to be used the most, perhaps because paladins are LG and because of that from time to times there's a wierd notion that pops up of LG as the goodest good there is.
A book on Aasimon (angels) would be interesting, especially because they're not beings of Good first and foremost, rather they're beings who express the will of their deific patrons primarily, with good oftentimes being part of that, but not always, and it can get ugly if you're on the wrong side of a deity. An exploration of how they vary by what god they serve, what pantheon they serve, etc would be interesting, as well as how aasimon beholden to different gods interact, especially when those gods stand opposed to one another or as rivals for worshippers.
