I had no idea this was part of a series, there are a few things that make more sense now that I know.  Or at least, it makes sense why they didn't make sense! Over all, reading this as a stand alone was fine, there were just some references, some relationships that could have improved with some context. That being said, I intend to go back and read the other 7 books in the series, I enjoyed this one so much!
There were some obvious nods to Agatha Christie, the characters in this book even reference her. I thought the setting was so unique, a castle in the middle of nowhere, housing spies that know too much to let loose in the world. There were so many quirky characters, dealing with the trauma of what they experienced in the war, it took me a bit to keep them straight.  But never fear! They started dropping like flies, so it mattered a little less in the end.
I really enjoyed this book, I never got bored and honestly was pleasantly surprised by how little I could predict who-done-it. A great addition to my must read authors and series!