I wanted so badly to like this book. While I've only read a couple of books in the Slayer's Guide series, I have always liked the idea of the series; I like most of the choices of species. The first Slayer's Guide that I read was The Slayer's Guide to Trolls, which I found extremely exciting, as it did so much with the trolls' regenerative abilities that was unexpected--like reattaching severed limbs in places other than where they came from and letting the original limbs grow back in place: presto, extra limbs! In fact, it excited me so much that I was immediately inspired to write a Slayer's Guide of my own.
Then came The Slayer's Guide to Duergar. Oh well. I will say that while I do have some complaints that can be leveled directly at the authors--the prose is pretty amateurish and repetitious in spots, the "atmosphere text" is downright painful, there's little here to inspire, many generalities, and a lot less crunch than the Troll volume, which positively dripped with ideas--my real complaints can be lain directly at the feet of the publisher, Mongoose.
The editing appears rushed and a little slipshod. At one point the authors mention that the eating habits of the Duergar "will be discussed at length below." The topic is never mentioned again, except that a mushroom farm is mentioned in passing in the included scenario. Did the authors forget? I doubt it. It seems more likely that the editor(s) cut it for length (and who could blame them--I don’t really need to read "at length" about Duergar eating habits, I can connect the dots myself), but then never bothered to line-edit the reference to the text to come. It's not a war crime or anything, but anyone who didn’t know better would blame the authors.
The art and design conception is downright silly. The authors, as another reviewer pointed out, thrash the fact that Duergar are emaciated practically to death, but do we get that in the illustrations? Other than on the cover, no. What we get instead is normal, muscular dwarves with mean looks on their faces. "I'm a snarly, angry dwarf! I'm EEEEEEEEEEvil!" My absolutely favorite incomprehensible art choice, though, is this. The authors introduce a new and interesting weapon: the bladed crossbow, which, if I understand correctly (though the description is pretty unclear) is basically a crossbow with a bayonet. It wouldn't work as described, for reasons that WILL be discussed below. However, after introducing this intriguing new weapon, does Mongoose commission a piece of art to illustrate it? No, instead they give us a drawing of a spear (carefully labeled "Spear"), a light crossbow (carefully labeled "Light Crossbow"), a club (carefully labeled "Stone Club"), a dagger (carefully labeled "Stone Dagger"), a battle axe (carefully labeled "Battle Axe"), studded leather armor (carefully labeled "Studded Leather Armor"), and a leather gauntlet (carefully labeled "Leather Gauntlet"). Thanks be to GOD, Mongoose! Being a complete moron, I had NO IDEA what those things looked like! Taking up a quarter of a page of a 32-page book for this was SO necessary! Again, thank you SO much!
But back to the bladed crossbow. The authors say "a vicious iron blade [is] mounted to the stock in front." I think this is impossible, because the stock isn't there for cosmetic reasons. It's there because a crossbow string has a serious pull, and the foot is put in the loop of the stock to allow the user to use his body weight to add leverage when cocking the bow. This added leverage would be particularly important for a bowman with short arms, like a Duergar, for example. Add a blade to the front and the blade would be quickly ruined by repeatedly being thrust into a cavern floor, and the bowman is likely to either keel over trying to use it or have to hop around on it like some sort of deadly pogo stick. Don't set it down on your foot by accident...
There are a couple of prestige classes, one a sort of stone druid (but with 12-sided HD), the Stonecaller, with specialization in stone manipulation and earth elemental summoning and control, the other the Black Rock Mage, an arcane spellcaster with abilities tied to black rock, a mineral of the Underdark with magical properties. The latter seems really overspecialized and likely too weak to work as a PC, the former seems, if anything, a bit overpowered.
In conclusion, I think that the problem is that despite the fact that every race in the Monster Manual does not merit a 32-page book, they get churned out anyway. Overzealous authors wanting to go on and on about mundane aspects of a species' life and overzealous editors constrained to the 32-page format appear to have combined the worst aspects of spreading the material too thin and cutting things out for length. I cannot recommend this book, given that much of what is new--cultural info, which boils down to "they're like dwarves, but more evil," tactical info, all of which seems obvious, etc.--seems extraneous, and given that (good, official) information for playing PC Duergar already exists or will likely be in official products (like Savage Species, or more likely Races of Faerun) soon to come, this book is superfluous. I paid five bucks for this on Ebay and I still feel a bit ripped off. Buy The Slayer’s Guide to Trolls instead if you want an interesting monster book, or buy Hammer and Helm or Heroes of High Favor: Dwarves if you want a good dwarf book, or buy Plot & Poison if you want a good Underdark book. If your budget is limited, hold out for Savage Species. It's $30, but it's official, and you know the art will kick ass. Likewise Races of Faerun.
Final word: love the series, didn't like this book.