Although it's met with mixed reactions, this book fills a much needed roll in the new World of Darkness. As the name might imply, it covers spirits... and all things related to them. Only instead of being focused on a particular gameline like, say Werewolf: the Forsaken, the information is much more general and can easily be applied to anything from mortals to vampires to changelings or any other monster. As such, gaming groups that don't have access to Werewolf or Mage (both of which already contain extensive information on spirits) will get just as much usage out of this book as those that do.
The first chapter opens up with Invisible Lore, detailing how spirits interact with mortals and the mortal world, and how human perception color encounters with these alien beings and abstract representations of concepts. It includes a number of sample characters who, with only a little work or re-imagining, could easily be used as NPCs in any game. You've got people who have encounters with djinn, devils and even UFOs, a 'good ol' boy' who thinks he's communing with ghosts of former Klansmen and pagans who think they are paying homage to the Old Gods, monsters under the bed and Hermetic occult systems trying (unsuccessfully) to learn about the Shadow, and folks who inadvertently have become beacons to the spirit world. Even suggestions on how to incorporate tainted locations or use the material with Hedge Magicians from Second Sight. Very good stuff.
Next it moves on to detailing the Shadow itself, and it's denizens. For those who don't have access to Werewolf or Mage, it fleshes out (and expands upon) the material from those books, explaining what the Shadow (spirit world) is, the Gauntlet that seperates the two worlds, Verges, Loci and Blights, and how the Shadow differs from the deepest wilderness to the heart of the urban jungle. Not only this, but it gives some sample locations to use in your games, and methods for mortals (and vampires, etc) to cross INTO the Shadow, and the dangers of doing so. Moreover, it also gives details on how to flesh out spirits and information on their predatory spirit-eat-spirit hierarchy. Great stuff for any game, but the most valuable part is that it includes information on how many supernaturals and their powers interact with the Shadow. That alone is a huge plus for the book.
From this, we jump to a chapter crammed full of useful mechanics. The book includes several new Merits and a couple of Flaws for any games, be they mortal or supernatural, dealing with spirits and the spirit world. Werewolves and Mages aren't left out, and in fact, the former get a couple of new Rites and Fetishes, while the latter get some new Artifacts. Cool but not ground breaking. But for integrating Vampires... well, theres a whole new Discipline called Blood Tenebrous, designed specifically for interacting with the spirit world. Although it is assumed to be a rare Discipline that theoretically any Vampire could learn or develop, the book also mentions it would be appropriate for a Mekhet bloodline (which is not explored further). But the real juicy part here? A new type of Merit - Cursed Items. These are objects that have absorbed spiritual resonance over time, usually quite bad, and can create certain effects... always with some drawback. Think of the monkey's paw or bargains with the djinn and you've got the right idea. Again, very cool.
The final chapter, which is probably the biggest in the book (and justifiably so), deals with spirits themselves. It contains detailed character creation rules for spirits, much moreso than Werewolf or Mage have given, including making them powerful appropriate to their rank and designing their bans, and the Numina. There's easily 60-75 Numina listed in the book, expanding greatly on previous material, so even Werewolf STs will get quite a bit of use out of this chapter. Moreover, theres also a 'spirit bestiary' detailing various things you can throw at your players. Of course this includes some things you'd expect... nature spirits, elementals, spirits of abstract concepts like gluttony and envy. But theres other stuff too. The Hammerbull was freaky, and the idea of elementals of void/nothingess was brilliant. It also includes a fair number of Claimed, mortals who have been taken over/possessed by spirits. And not just humans either, but also animals and inanimate objects. Alot of twists here.
I think alot of people running Werewolf or Mage games might, understandably, be somewhat apprehensive about this book. Nonetheless, it greatly expands upon and clarifies the previous material on spirits, so Mages and especially Werewolves might get even MORE use out of this book than mortals, Vampires, Prometheans, etc. There are alot of twists and cool ideas that break outside of the box, so it'll keep them on their toes. Even so, the other game lines will still get alot of usage out of this book too. In fact, given that they are much more limited in their interactions with the spirit world, it may even make them that much more horrific. Pretty much anything you could ever want to do with spirits is covered in this book, and better still, theres alot of inspiration to use too. Its an essential book for any World of Darkness games.