Summerland by Hannu Rajaniemi

Hannu Rajaniemi imagines a world that diverged from our own, not into the brass technology of steampunk, but into a world where the dead can be communicated with by Marconi's devices, a world of ectophones, ectotanks, and spirit cages. 

Take that backdrop and mix it with a pre-WWII tale of espionage, and you have a story that keeps you guessing until the end. The two main characters weave a braided path through the narrative, each commanding the reader's sympathies though often appearing to be on opposing sides. The world of the spy, where nothing is what it seems, is mirrored by the world in which it is set, where our history has been turned on its ear by contact with the land of the dead, yet still manages to play out most of the conflicts that marked our history. The story is told with a minimum of explanation; one of the pleasures and one of the challenges of this book is figuring out how this world works, how it diverged from our own, and how some of our historical figures would have reacted in these game-changing circumstances.