The first couple of chapters talk about the book, including introductions by the two main writers - who show a lot of enthusiasm for the project. Not just the "Hey, isn't this a great idea?" kind of self-aggrandising enthusiasm, but a love of what they're writing and a clear joy at getting to write it. It's something that's not seen to often in big-company games now, which is a real shame.
The next chapter is pretty awesome - "Overview of the Underdark". Here, you'll find a brief (yet surprisingly full) description of the processes that occur underground, and the various ways these form into dungeon features, and ways to use them. Form the simple stalagmites and stalactites, through underground lakes and lava pockets, deadly gas build-up, and cave-ins, these descriptions paint a vivid picture of the dangers one might face from the environment alone, never mind monsters! A lot of these features will be going into the next dungeon I write...
Next is a couple of chapters dedicated to new rules covering the ins and outs of dungeoneering, from movement rates for various conditions, fighting in enclosed spaces, using ropes as levers and walkways, and a whole host of Non-Weapon Proficiencies to round things out. These are great, ranging from hunting and creating weapons, to things like Fungus Identification and Blind-Fighting. To me, these add an extra layer of specialisation for characters who intend to spend most of their time underground - sure, not too useful in a city-based campaign, say, but if you're about to head through a full-campaign megadungeon, these little tricks could save your life.
(Interesting Trivia Time: Did you know that non-weapon proficiencies were only introduced to the game by Oriental Adventures? I didn't.)
The equipment section is where this book really stands out. It has everything you could think of taking into a dungeon with you, and quite a few more that I wouldn't have expected (who expects to need a fold-away canoe in a given adventure?!). Each piece of equipment adds some extra rulings, or adds to existing ones in interesting ways. While these are mainly useful to dungeon crawlers, most of the equipment could easily find another use to clever PCs (especially Thieves, as a lot of the stuff here covers stealth, mobility, and bypassing problematic obstacles).
The section covering monsters is interesting - it's got some information about the various humanoid races that live under the earth, from the classic Drow, to the little-used Duegar and Derro, the aquatic Kuo-Toa, and the abominable Illithids and Aboleths. Each of these only gets a bout half a page, but they make a great overview of the objectives of each of these races, and a great resource for new GMs.
Finally, the book contains a whole setting, a nice mapped-out chunk of the Underdark for the taking.
(Again, interesting fact time - this book is the first official use of the phrase "Underdark" in D&D literature. Neat! Next, to the Shadowdark!)
The maps are fabulously detailed, but still leave a lot of areas relatively free to interpretation. Some of the set-ups (like the massive "drain", controlled by Kuo-Toa who can change the flow of water into the lower levels) are fantastic, and even without set encounters, can provide a wealth of cool things to do in the setting.
The maps are done in a strong, isometric style - it take some getting used to, but the effect is great for conveying a truly multilevel area, rather than a "staircase down to the next level" type deal. They can, however, be a little bit cluttered, and the isometric view doesn't help any there.
All in all, a very strong book, even to modern eyes. There are one or two little niggles with the rules, however... but I'm still planning to use it when I finally get around to writing that mega-dungeon...