The German experimental subjects are monsters, but also victims of the moral monsters that created them; there's plenty of superscience cruelty to ground the (equally real) metaphor of the monstrous Reich. And then, on the British side, we have blood magic; and a plausible rendition of where the "keep calm and carry on" spirit would take a nation facing a war far more bitter than the Blitz of our history.
But this is not a war book. It is, I think, a hero-vs-supervillain book. (The hero is not a superhero; the supervillain is not who you think it is.) The sequel will -- of course -- be set in the Cold War (of course the Soviets get superhero technology), but I'm not sure what kind of book that will be. One would like to imagine the hero of this book gets a happy retirement, but...
Nasty magic and secret agents of course adds up to Tim Powers, and I think the comparison is valid, but not for clever history-twisting. (History is flat-out derailed, starting with pyrokinetics burning a fat hole through the Maginot line.) It's the spirit of people's lives getting horribly and almost irrevocably wrecked.
"Gripping" and "page-turner" are cliches, but yeah, this book's got it.