4/32 of my Dark Sun series, and in some ways, the end of the first wave of Dark Sun material. That is, I've since learned that Dark Sun started as a war-game setting where everything would be fighting everything and everything would be powerful. Though the concept changed as it went from war-game setting to rpg campaign, that general outline is still visible; and this is the book on high-level play that fulfills the second half of that promise.
As the intro notes -- and more rpg books should continue to have intros that describe what the book is trying to do -- this book presents rules on high-level play that (a) defines the world of Athas (the world of Dark Sun) and (b) might have gone in the first core box, only there wasn't enough room (which is a good reason why some of this stuff is here) and it might've spoiled the novels (which is not so good a reason why some secrets were held back).
So here's the big reveal: in the core book, we learned that Athas is a resource-poor world where over-use of magic has killed the world; and there are very powerful sorcerers who control the seven remaining cities and sometimes a dragon rampages around the world. What this book reveals is that those sorcerers are slowly being transformed into dragons through high-level magic and that the rampaging true dragon is actually a sorcerer who completed the change.
That's all very interesting, and I kind of feel like that info should have been in the core, or maybe in some additional book that was like the secrets of Dark Sun. (Note also that this book has info on the sorcerer kings, but there's still a whole slice of Athas's history that isn't yet revealed to the DM/players.)
But instead of just being "secrets of the world," the remit of this book is "high-power play for all characters," and that leads to a sort of lopsided discussion. That is, magic users who reach 20th level can decide to switch classes to psionicists, and when they reach 20th level in that, then they can transform into magical creatures depending on their magic. (That is, defilers turn into dragons, of which there are at least 8 in the world; preservers can turn into good avangion, which are angel-insect-looking creatures, of which there is maybe one?) Then you can start casting really epic spells -- and, in fact, half of this book is new magic and psionic powers.
(Sidebar: the intro also notes that they wanted spells that weren't just "more powerful fireball" but truly world-shaping. I am not sure they met that standard in the spells given.)
But what about non-magic classes, like your fighters and thieves? Oh, they just get better at what they do, or maybe gain new powers like the thief's ability to... bribe officials? Why is that a power the thief can only gain at 21st level? Really, the non-magic classes are really underdone in this book, especially since something like the fighter's ability to gain followers is really treated like a problem to solve (how to put the focus on the PC rather than their followers?) rather than a power or opportunity.
This book also fulfills the original vision of this as a war world by adding in stats for various vehicles and armies, which aren't very interesting to me (this is all back when TSR was using their BattleSystem rules for war), but I do love the illustrations still.
So I think there's some good ideas and intentions here, but I question why those ideas are fleshed out here, and think that the intentions weren't entirely fulfilled. The book also claims that these high-level rules can be used in other worlds, but I doubt that, especially the most interesting rules about defilers/preservers; and even the best parts of this book are probably the little snippets of Dark Sun legends and stories.