Part II of my review:
Apart from the vast diversity of options (none of which are neglected or considered superior), the sheer attention to detail regarding the finer points of conspiracy-creation and the like retain their validity even beyond the confines of this game. Oh, and then there's massive array of supplemental material, the fact that you literally can derive so much awesomeness from this book. If you play GUMSHOE, any GUMSHOE game, and always felt like the engine had more to offer, then you should consider this a must-buy book. If the theme even remotely interests you, well, then this should be considered a unique and rewarding game to play. Night's Black Agents is, by any measure I apply, a superb game. My review may not reflect this 100%, but I tried VERY hard to pick this book apart - but quite frankly, there is nothing worth complaining about. Sure, its combat will never attain 13th Age's or PFRPG's level of complexity. But neither will those systems ever come close to the investigative caliber of this book.
If you're looking for a change of pace, for vampires in your GUMSHOE game, for a glorious investigative game, for a rules-expansion of the highest caliber, for any of the above virtues- then there's no way past this book.
My final verdict will be 5 stars + my seal of approval, accompanied by being tagged as an EZG Essential-book for GUMSHOE. Once I've reviewed enough books of the system, I will provide the corresponding Essentials-list.
Reviewed first on endzeitgeist.com, then submitted to Nerdtrek and GMS magazine and posted on Lou Agresta's RPGaggression and here as well as on OBS.