bought this book day one, and i have read through it 2-3 times since i have gotten the PDF, i won't be talking about the hardcover, because as of writing this review, i haven't gotten my copy yet.
i'm sure i am not the only one who are running Crossover games, so majority of us have homebrewed rules to run things games, with some success and failure.
this book is a solution to give you rules and setting on running a crossover game to have it makes sense, and have a bigger evil for all to attempt to combat or survive. the Contagion is ambiguous in how it was created, whether it was a virus created by the God-Machine a little too well thus infecting the God-Machine, or you can make a super flu that has a 99.7% mortality rate specifically infecting supernaturals by making their touchstones forget them, human blood burns them, really whatever makes sense for your setting. based on this premise of a super virus gives me Stephen King's The Stand vibes. it gives you two major factions and sub sections of the factions, the Sworn are those who try to contain the Contagion, while the False wants to use it for any means. there are some interesting sections and vector powers, which is basically powers that every supernatural and mortals can use, giving specific abilities based on what you are playing.
now the big question is, do you need this book to have crossover games? simple answer.... yes and no.
yes because it gives you rules, setting, and a big unknown baddy that all supernaturals and mortals to come together to work alongside eachother and combat it. however, if you have a homebrew rule setting that makes your chronicle work, then this book isn't for you. however, it does have rules that you do not need the Contagion itself, it can be something else, and it'll fit your chronicle. really, it is a conpendum/supplemental book, much like Hurt Locker, however, it will add to your world, and like i said, if you don't get it because you have a homebrew rule that works, thats fine. if you do get it, it is a good read, and gives you great ideas on how to incorporate the Contagion and the Sworn and False into your setting.
i'll end this by saying, this is primarily for Storytellers, players do not need this book. players just need their character specific book, so if you want to play a Vampire, Werewolf, Mage, Geist, etc. get the specific book, jump in and play in this setting/rules.
[5 of 5 Stars!]