If you're looking for a whole slew of new rules or powers for your Wraiths, you probably won't find much in here that'll help you. It's got a few new Arcanoi alternate arts and a few artifacts and new skills that are quite neat and very much worthwhile for campaigns, but those are not by any means the focus of the book.
Rather, this book is all about the fluff. It tries to portray in a sympathetic light a system that has grown largely corrupt and inefficient as a necessary evil. And unless you're already completely biased against the Hierarchy, it does a pretty good job.
The book has an entire chapter devoted solely to the perspectives of various Wraiths (written down as blurbs) talking about the development of Stygia and major points in its history.
The book also has a section on what Stygia's all about, including a little extra info on the Deathlords and their original purpose, and what Wraiths do with their spare time. They also include a little bit about how Necropoli fit into the picture, and list a couple of unofficial legions that comprise the elite of the Hierarchy's servants.
They even have a set of plot hooks for some of the secrets of the Hierarchy, and the official (and unofficial) standpoint of the Hierarchy on other Wraiths, mortals, and supernatural creatures.
If the Hierarchy is going to be a big part of your Wraith campaign, you can't go wrong with this book. If the temptation of a few new abilities is singing its siren song to you, you might like what's in here, but there won't be too much.