First, know that what is in this book is pretty good, as we’ve come to expect from the Chronicles of Darkness. The low star rating is not about that at all.
The descriptions of the various multi-splat groups that fight the Contagion – divided between the broad categorization of the Sworn and the False – are given well thought out histories and visions, and their various powers, called Vectors, work well and can add great options to any game. The fact that each Sworn faction comes across a little too much like a heroic squad of monsters dedicated to save the world from reality-shaking Contagion does go against some of the themes of various CofD games, but this book does not mandate such an approach. Neither does is depict the False as only nihilistic fanatics, but it's plenty possible to fall into thinking of these two sides in such terms.
Next comes a history of the Contagion and a chapter devoted to running these tales with multiple types of characters from across the CofD titles. This is where the "What's here is good, but..." really showed up. The chapter for the Storyteller fails to give enough guidance on running multi-gameline stories, and the piece I miss most is not the balancing powers or the way various character types would interact. Rather, I would have appreciated more discussion of the themes that each game relies upon, thoughts about how various groups of different types (vampire covenants, fae Courts, hunter Compacts, etc.) can find common ground, maybe ideas to manage how inherently antagonistic supernatural beings' hatred can serve a chronicle. None of this shows up. The chapter almost feels like an afterthought when compared to the word count in the rest of the book.
The entire second half the book is a travelogue of sites where the Contagion has taken hold. These would work better if they’d been offered in more of a modular fashion, allowing a "toolbox" pick-what-you-like approach, but with a little work, that's still an option. However, it's here that the one biggest failing of the book comes to the fore.
Onyx Path Publishing advertised this as a crossover chronicle book, and it is exactly that, but it's not crossover friendly at all. If you are familiar with all of the CofD lines, this book reads pretty well. However, if you are not so well versed – note that at the time of this writing, two of the lines (Mummy: The Curse and Hunter: The Vigil) don’t have Second Edition versions out AND Deviant: The Renegades is still long off – The Contagion Chronicle becomes confusing at best. If the book included a few pages of description on each game line (its themes, what the beings call themselves, how they divide themselves, some words on how their powers work, a brief glossary of common terms), it would make the experience much easier. On a single page, for example, the word "beast" may show up with little to delineate if it refers to a changeling seeming, the inchoate passions of a Kindred, one of the Begotten, or simply an outsider's view of a raging werewolf. Even being fairly familiar with those games, I found myself confused at times.
The Contagion Chronicle is a good book that could have been great with a little more effort. If you want or need some great ideas for crossover stories to tell, definitely buy it. If you are looking for rules and help with running crossovers? Skip this one.