Given that the default starting location for many Dark Sun campaigns will be the City-State of Tyr, this particular accessory is excellent (but not quite *required*) reading for the initial phases of adventuring on Athas.
Temporally, this campaign expansion book is firmly planted in the post-King Kalak era when Urik mobilises for the iron mines, the council takes over the running of the city, and the Veiled Alliance think about going public. That said, its not all about politics. The books itself is a very thorough description of the city.
The first chapter starts out by looking at the history of the city and detailing what life is like in Tyr. Even down to the fact that they're pretty nice when it comes to the distribution of water (you can have a ration of it for free!). The details and politics about the governing body and King Tithian follows and how the various factions are at play on the council. Here it gets quite detailed about the politics and delves in to detail about traders (who now use the arena), justice and crime, and what the Templars are up to now they no longer have their spells -- and would you believe some of them are helping to direct civic projects. Surely they have their own reasons, right? (hint: They're probably not doing it out of the goodness of their hearts and are just as brutal as they always were…).
The physical description of the city and the local environment are covered in the next two chapters. This includes, but is certainly not limited to, details about farming, architectural details, where various classes of citizen live, and of course: a full colour fold out map for help with the campaign. The districts are described in plenty of detail (and certainly sand-boxy enough to still do things with) should the PCs wind up in the trading district and so forth.
The next two chapters cover the Golden City and UnderTyr. Of the former, there are few surprises and precious little is advanced over previous material, but of the latter, a great deal is revealed. The labyrinth of catacombs and pre-history Tyr buildings is (to me as a GM and PC) exciting to see there. I would have loved for there to have been more detail on the various locations to be honest, but there's enough there to generate some unique encounter sites for the PCs to play around in.
The final few chapters concern themselves with NPCs, campaigning within Tyr and how different classes are represented and interact within the city-state. The psionic school is a stand out here for me, but it also talks a bit about religion (or lack thereof, given that there are no deities on Athas, hence some druids a talked about). The adventure suggestions are reasonable, if not outstanding, within these chapters and provides some good ideas should the GM decide that the regular progression of adventures is not to be used.
Overall, it scrapes a 4 out of 5 stars from me (it was almost 3). I think the sheer level of detail and expansive range of coverage makes this campaign expansion book high quality. But equally, if one is not gaming in the Tyr region much (or only to begin with due to, e.g., undertaking the Dragon's Crown adventure), then its usability might be less.