I thought this book was pretty good. There don't seem to be nearly as many books written on angels and celestial beings as there are on the fiends, so I feel it is a bit more unique tha, say, Guide to Hell. It contains a few new locations on the Upper Planes that are somewhat interesting, including the land of the Quesar, a testament to how angels aren't always perfect or just (not in the Quesar's eyes, anyway). It also goes into celestial motives and lifestyles to some extent, mentioning that they don't actually spend every last waking moment crusading for the cause of good; sometimes they are content with relatively simple and mundane pastimes. The book gives information on Aasimon, Archons, Asuras, Guardinals, and Eladrin. The info isn't super detailed, but to me at least it is somewhat new. There are also huge sections dedicated to creating celestial PCs, which include most forms of each celestial race except some of the most powerful (notably solars and tome archons). I think that these are pretty well done; it has most of them start off weaker than their Monstrous Compendium counterparts and gain more spell-like abilities, magic resistance, etc as they advance. One thing I don't like is that, like pretty much every race in 2e DnD, they try to balance them with humans. Even Aasimar, human/angel hybrids, are balanced against humans. It really makes no sense to me why something half human and half angel, which should by all rights be quite a bit more powerful than humans, still average out to being about equal (nowhere does it give any justification in the book as to why Aasimar should take a -2 constitution hit, but I guess they just have to have something to balance out their +2 wisdom bonus, even if it doesn't actually make sense). That kind of "balancing" just seems extremely forced and unrealistic to me, but I guess that's more a gripe against the 2e system in general than this product in particular. Oh yeah, except for the few new sites at the begining, there really isn't any info on the planes themselves that the Celestials live on. You'll need Planes of Law and Planes of Chaos for those. There isn't any real detailed info on any of the gods they serve either, though there is a condensed list in the back that names a lot of Powers and their alignment and realms of influence.
Overall though, if you're interested in the fiend's good counterparts, this is a pretty good book.