Prince of Thorns, King of Thorns) is all grown up, and more complicated than ever in this final volume, Emperor of Thorns.
And as the title and the cover give away, he's going to achieve his ultimate ambition, of re-uniting the very post-apocalyptic neo-feudal Broken Empire (which is pretty much what is left of Europe plus a chunk of North Africa) under his rule. We've pretty much suspected this was going to happen from the beginning, because Jorg is one ruthless little monster and he always gets what he wants, even if he has to kill people he likes or loves in order to do it.
So the question has always been, not if he will become Emperor, but how. And the how has been a most tantalizing, challenging question, because the traditional way to the Imperial Throne is not unlike that to the throne of the Holy Roman Empire of days gone (very much) by; there is a set of Electors, kings and dukes and lords of the empire's constituent lands, who must come to a consensus on who is to be the boss. If they can't, no emperor. Ususally, they can't, for a number of reasons, chief of them being 1. Instead of just a handful of electors, there are 100 and 2. Since the imperial throne has been empty for over a century, the imperial court is pretty much just a big empty city of ceremony full of officials and functionaries and guardsmen (the imperial guard is the chief sink for the empire's ambitious second and third sons who don't want to become priests), leaving the kings and dukes and lords with most of the real power, which they do not want to give up.
King of Thorns dealt largely with Jorg's ruthlessly clever elimination of his greatest single obstacle to achieving his ultimate ambition, but left our man more unpopular, feared, etc. than ever before. But the Imperial selection process is pretty much a popularity contest. Oh noes!
So lots of Emperor deals with politicking, though not as much as I'd expected. And what there is is pretty indirect-seeming and circuitous; Jorg makes friends chiefly via saving his enemies from bigger and more powerful enemies, meaning ALL THE SUBPLOTS YOU GUYS, but said subplots go a long way toward seriously enriching our understanding of Jorg (again, via a whole chunk of the book taking place "four years ago", still set after all the events of the prior book, but four years earlier than the most immediate narrative of Jorg's pilgrimage to the Imperial City for the vote), who emerges as an even more complicated figure than he already was as the usual transitional events leading to adulthood leave their marks on his already very marked-up self.
The same is true of his world, about the nature of which we learn even more as we follow Jorg's progress through it; not only in terms of its geography and inhabitants, but of its nature as a post-apocalyptic feudal mess, still littered with "Builder" technology and artifiacts and, as we learned in the second novel, haunted by Builder ghosts.
And yes, by the end of Emperor, the urge to go back and re-read from the beginning is strong; you will have a completely different understanding of the driving forces of Jorg's life and character and probably have a different take on the narrative itself.
And yes, the ending is satisfying in almost all of the ways we want it to be, and the way that it isn't is just fine, really, because one of the major points of Jorg's story is the hollowness of vengeance, after all. Bad things happen; punishing the baddies doesn't make them un-happen, it just uses up a lot of your time and energy that are better spent pursuing more important goals.