This was highly impressive.
I grew up playing DnD 2nd Edition and Star Wars 2nd Edition as well as GURPS and other systems. Sometimes I find my nostalgic curiosity gets its hooks into me and I just read a gaming book for kicks. (My wife and I have recently gotten back into table top gaming)
Witch Hunter: The Invisible World is simply a delightful setting. While the stories and campaigns are only as good as the participants, there is an amazing groundwork here that goes into tremendous detail. Ever faction and idea in this book is so well rooting into the real world and the religious history of imperialism that I found myself learning about history as well as applying my past knowledge of history to enrich my idea of the setting.
The combat system is lethal, the magic system covert and under-powered, and the social/diplomacy dynamic is very flexible to meet the needs of the GM. I would say that the mythos behind the game is so steeped in history that it may be prohibitive to those uneducated in the churches' imperial domination. This I saw as a bonus, being personally delighted in employing my prior knowledge but for some it may be a deal breaker.
The book is not perfect, however. The font is a bit too small for a book that you must reference constantly on the fly, and the artwork is not consistent and feels rushed in spots.
Overall one of the best gaming settings I've encountered, and when read as something to purely enjoy it is captivating.