OK... the original purpose of D&D was to crawl through dungeons while killing wandering and other monsters and finding nifty pieces of equipment. The game has come a long way since then, with hundreds of core books and supplements through different editions adding layer after layer of complexity to the D&D fantasy world.
Demons and devils were defined; evil was given its own spells, magic items and races. The good guys, for their part, had morally limited and easily stereotyped paladins and the occasionally pure-hearted cleric. That was it. There was no real reason for a fighter, rogue or wizard to be any more good than their alignment description read. The Book of Exalted Deeds changes all that.
Good has been given power, real power, and is now just as capable as evil of showering benefits on its devotees, though at no less cost to those devotees.
This book opens with a discussion of the motives of good; which acts are good, which are not, and the exceptions to the rules. It's never a problem for me, but this section would be quite handy for those DMs and players who have trouble figuring just what a character's alignment means in practise.
Also included is the idea of being "exalted". This isn't getting on a moral high-horse or anything of the sort, it's simply the idea that just as some villains can be despicable beyond human comprehension, so can heroes be righteous to the same degree.
Next we get to the meat and drink of the book: the new stuff. The magic items are adequate, not much more. There's only so many new adjectives you can add onto the beginning of item names, and only so many powers you can give, but at the very least this book includes special enhancements that directly counter enhancements from the dreaded Book of Vile Darkness. The same goes for feats and spells, really, although some completely new concepts are also entertained.
The book really comes into its own with the prestige classes, monsters and descriptions of greater creatures of good. In these sections, you're given some specific statistics, but you're also given a really good guideline for just how you can create your own special and sacred servants of the eladrin, the angels and the guardinals. Various otherworldly and mortal servants of the three great bastions of good are described, including a lot of the eladrin that were missing from previous supplements. The tulani and the firre were particularly appreciated. The greater creatures of good, the angels, the high eladrin, the greatest of the guardinals, are described in detail, with thorough descriptions. Given their tremendous power, I'm a little surprised that they weren't generated via Deities and Demigods, but that's a minor quibble.
These are the chapters for your perusal:
Chapter 1: The Nature of Good, pages 5-23
Chapter 2: Variant Rules, pages 23-33
Chapter 3: Exalted Equipment, pages 33-39
Chapter 4: Feats, pages 39-49
Chapter 5: Prestige Classes, pages 49-83
Chapter 6: Magic, pages 83-121
Chapter 7: Celestial Paragons, pages 121-157
Chapter 8: Monsters, pages 157-191
What I liked best about this book is that, as opposed to the majority of WotC's other products, this book appears to be cleanly edited, neatly presented and well-organized. I'm not an organisation freak in my waking life, but when it comes to reference supplements, I really appreciate having everything laid out in plain, simple order. It shouldn't be too much to ask.