Man, how about that weather, am I right?
No, actually, I am not, in fact, right. It's probably okay for Valley of Dust and Fire, by L Richard Baker III, to talk so much about the weather. The world of Athas has a very different climate than Earth, and part of describing that world is telling us what the sky is doing. I gush whenever a book tells me what the clothes are like (this book does it too!), so it would be weird if I didn't want to know about the conditions in which those clothes are worn. It's just that there's a lot of it. It's pretty interesting - giant, miles-high dust storms, deadly tornadoes of flame, etc - but it's not until page 50 (out of 90) that the book starts talking about the City of Ur Draxa, which is the whole reason you're out here in the first place.
It's not as if the first half of the book is useless. The first chapter, about the Sea of Silt in general, could help DMs with any number of games where the PCs must brave this major environmental obstacle at the edges of the Tyr region. The middle two chapters, while far more specific, could probably be stripped for parts. Maybe there are Dead Forests and Shard Flats in places other than the Valley of Dust and Fire, or open stretches of lava unconnected to The Ring of Fire. Nonetheless, these places are not destination. You'll only ever be passing through.
I mean, you could probably set a whole campaign in the Silt Archipelago, a series of mud-flats that sprang up like islands in the silt, and which hold numerous villages which have never known the tyranny of the Sorcerer Kings. Although, if you did run that campaign, this book would only give you the broadest of outlines.
You could also probably do something with the Mountains of the Sun, if you're willing to rip out the racist presentation of the local dwarves (they "may be ignorant savages, but they possess a lot of unusual oral legends concerning the days before the Dragon.") They share the island with deadly giant spiders, and it's kind of intriguing me to imagine a dwarf economy based entirely on harvesting spiders and spider products.
Yet the real meat of this book is chapter 4 (so much so that it is the only part of the book referenced in "Chapter 5: Campaigning in the Valley"). It describes a new city-state and promises "detailed description of the greatest and deadliest secrets."
That last bit is a little oversold. We learn where the Dragon goes when not terrorizing the city-states of the Tyr Region (A fortress known as "the Dragon's Sanctum," which is most interesting for having a whole forest of Trees of Life, but no animal life "not even insects") and it's teased that there's something more mysterious going on, with The Black Sphere at the center of the Sanctum (I understand a vague outline of what's going on, between reading my friend's copy of Defilers and Preservers: Wizards of Athas and seeing online conversations about this very subject, but I can't say I'm impressed with what I've heard - that the Black Sphere contains an imprisoned wizard who is even bigger and badder than the Sorcerer Kings and the Dragon), but ultimately answers are few and far between.
The new city-state is pretty good, though. It's a grim, unpleasant place, suitable mainly for running games that are pessimistic even by the standards of the setting as a whole, but there are some unique social dynamics at work that could make for a memorable fantasy campaign. The Ur Draxans are a fanatical warrior culture with no one to fight but their slaves and their own outcasts.
Seriously, they are the most completely useless army you've ever seen, fantasy or otherwise. Their homelands are surrounded by a terrifying moat of lava and a permanent storm of ash that makes flight in or out nearly impossible. The whole thing is enchanted with dragon magic to make teleporting in or out impossible, except through the one dedicated magic gateway, the far end of which is in a blasted valley, surrounded on all sides by hundreds of miles of trackless silt that will swallow up and suffocate anyone who puts their weight on the surface.
I kind of like the pointlessness of the Ur Draxans. They are all nobility. Everyone who is not a slave is a landowner and a warrior-aristocrat in their complex feudal system. They are the army of the Dragon, but the Dragon doesn't need an army. In fact, the Dragon supports them, bringing back metals and new slaves that the city-states of Tyr give it in tribute. It's unclear why the Dragon does this (though I suspect it's an emergency precaution in case new metaplot makes its way to the Valley), but the overall effect is that it's impossible to even pretextually justify this society. They're just these nasty authoritarians that hang out in the middle of a dried-up ocean and believe themselves to be the last "civilized" people to survive in all the world, and despite having all the time in the world, they're too macho to crack open a book (this is actually a major area of friction in this society - they have templars and administrators, to keep the place running and do the Dragon's bidding, but choosing learning or the arts over warrior training leads to loss of status in their society).
Ur Draxa strikes me as a nice change of pace from the vague "always evil" societies we usually see in D&D (probably because the Ur Draxans are PC-allowable fantasy creatures - humans, half-elves, dwarves, and muls). As dysfunctional and cruel as their society can be, it doesn't seem outside the realm of realistic human behavior. They engage in the daily dehumanization necessary to be an aristocratic minority in a slave state and they're pumped up on legends of their own greatness ("the belief that he or she is a hero of a race of heroes.")
Overall, I think The Valley of Dust and Fire could make for a very credible end-game location for a Dark Sun campaign. Unfortunately, it's unclear whether the people in charge of the setting are still willing to commit to end-game campaign models. The part of the book that talks about PCs fighting the Dragon says, "If properly run, the Dragon is able to defeat even a party of the highest level adventurers," which is maybe a tad ambiguous ("oh, I get it, able to defeat, because it's a fair fight for a party using Dragon Kings material"), but later cleared up in the worst possible way, "If they're here to kill the Dragon, they are doomed." I suspect that what's happening is an attempt to hold the door open for the plot of the novels, and if so, that's really disappointing from a setting that has thus far been willing to cast the PCs as major historical figures. However, I'm not willing to declare myself betrayed just yet. It's only one optional sourcebook, not even about a major subject of interest (at least, going by the title). Plus the section does end with the hypothetical "What if they win," so it hasn't entirely lost the spirit.
Ukss Contribution: The Silt Drake is a giant serpent that lives in the Sea of Silt and devours any sufficiently large prey that crosses its path (it prefers silt horrors and giants, but will settle for a group of PCs). Like many semi-intelligent monsters on Athas, it has psionic powers. Weirdly enough, those powers include Clairvoyance and Precognition.
According to the description, it "uses its rudimentary psionic abilities to locate prey, above or below the silt," but it's never made clear why they couldn't just handwave the use of their physical senses. If the description said they tracked prey through smell, hearing, and tremorsense, no one would be interrogating that explanation. So we have to assume that being psychic is an important part of their whole deal, but like much of old AD&D, it's presented in a very matter-of-fact way that doesn't seem to realize that the weird thing is weird (I don't believe it's an intentional Dune reference, but I could be wrong).
I think I'll put them on Ukss' moon and give them a complicated religious relationship to the human seers that make the place their home (that will be an intentional Dune reference, though).