Opening with a fine horror story that rather made me wish I wasn't eating my lunch while I read it, this book is about the horrific things that can slither out of the Abyss and into the mortal world. Ordinary people are pretty much defenceless against them, so it's up to the Awakened to do something about any that they encounter... and theses things are rooted in horror, indeed may be the underlying reasons, the source for the horror stories, if not all the misery, in the world itself.
The Introduction explains this and discusses researching the things that come out of the Abyss - for without knowledge, one is pretty much defenceless - and describes how most of the rest of the book is a catalogue of the strange and unwholesome manifestations of the powers that lurk in the Abyss. It ends by suggesting suitable source material, starting with The Fortean Times and providing a reading list of horror stories and a selection of movies. One of the suggestions is H.P. Lovecraft, but not as you might be accustomed to treating his work: for these purposes concentrate on the strange unearthly manifestations that often ignore the havoc they are causing because the Earth and those on it are plain unimportant to them... that's how the creatures of the Abyss behave... rather than worrying about pantheons of ancient (and generally evil) deities.
Before we get on to the actual critters, though, there's a chapter called Otherworldly Dread. Primarily aimed at Storytellers - as indeed this whole book is - it looks at how to incorporate the Abyss and the horrors emanating from it into your chronicles. There's plenty of advice on how to use these intruders, making them an effective threat (and something downright scary!) and even how some twisted and perverted people seek to use them to their own advantage.
And then there's the creature collection. Each one is presented in a standard format, starting with the name(s) by which it is known in this world. There's scene-setting fiction, notes on how it appears to senses both magical and mundane, details of what is known and what it does, how it gets into the world, what it tries to do once there and the all important details of how it can be banished to whence it came. There are ideas and story hooks for getting them into your game, and any necessary game statistics you'll need when your mages square off against it. There are a full twenty-four of these unspeakable things for you to contemplate...
Horror may not be your thing, but even so it might be worth sparing use as a warning that being a mage is not all fun and games and working your will in the world. If you and your group do like horror stories, well there are enough here to keep you busy for a fair while. You might even run an almost X-Files-style game with a group of mages dedicated to hunting down and eradicating Abyssal manifestations wherever they raise their ugly heads.