OK, so making sense of this book can, on occasion, require a few extra seconds of thinking, but all in all it's a very interesting insight into the historical practices of the ninja/shinobi/whichever name you prefer. Now, I'll state straight up that, whilst there were things here that i'd not read before, much of the information given has been widely used in novels and other texts related to the subject area. What this offers that is different is an insight into the mindset of the "ninja".
Be aware that this is not a martial arts book! It does not say "if X does Y, then you do Z". It does not have cod drawings of stick men throwing each other on the floor. What it does have is information on psychological, social and even engineering practices of a group of people with very specific needs.