I read cozy and historical mysteries, a bit of Paranormal/UF, and to mix it up, I read science and gardening books on occasion.
Not bad... a fairly interesting overview of those places that have existed or still exist that are hidden, or secret, or just very, very private. Well researched and well organised, but ultimately failed to deliver; the information was good and interesting, but little of it made me think "wow!" or feel compelled to torture MT with "listen to this!" excepts read aloud when he was trying watch the soccer.
I was most disappointed by chapter 4, a chapter almost entirely given over to Wewelsburg Castle - Himmler's "Black Camelot". I wasn't expecting, nor wanting, graphic details, but the authors hinted in the introduction that what happened here at this castle was what made it possible for all those people to commit themselves to the horrific atrocity that was the holocaust. When I finally got to the chapter itself, there was no information at all about anything except a description of the castle itself and vague references to the Jehovah's Witnesses housed at a nearby camp that were forced to do all the labor on the castle renovations. I'd have liked some kind of information, or even speculation, about how Himmler was able to turn these men into monsters.
Other chapters, though, I found chock full of new information (to me); I learned a lot about the Knights of Malta, one of the last remaining Chivalric Orders, and there are more than a few new places on my "someday I need to see this" list.
Overall, a good book if you like this sort of thing and you're able to find it used at a great price, but at full price it might be found to be lacking in some areas.