Priest is a British SF writer. I think I've read all his books. His style is quite unusual: quite restrained and - this is a funny word to use - humble. His characters often don't really understand what's going on, and they find it hard to take control of events. His most famous book, the Glamour, is about people who are so self-effacing that they disappear from the world altogether. The writer he is most like is Kazuo Ishiguro, I think. And like Ishiguro, shocking violent and wicked things do happen in his books quite often, but you are left almost as bewildered by them as the protagonists are.
This book is based on a cliche - an alternative history where Britain and Germany ceased hostilities before the US entered the 2nd World War. Not sure if non-Brits know this, but Rudolph Hess, Hitler's deputy, flew to Scotland in 1941 (? I think) to sue for peace and was incarcerated until 1987, when he died alone in Spandau prison. More intriguingly there is supposed to be convincing medical evidence (for real) that this 'Hess' was actually a double, with a mental illness. If you are interested, there's plenty of conspiracy theory stuff on this.
Priest takes this idea of the double, and the idea of alternative history, and creates a subtle story where it appears that some people are coming unstuck between at least two alternative realities, a process which is conflated and blurred with the concept of a 'double', a person who for one reason or another looks exactly like another. Towards the end layers of multiple story, and the moral ambiguity of peace and war, build up into lucid dreams within dreams. Some characters, like Churchill, appear to be able to understand what is going on and even to take charge of it, but everyone else is adrift.
I recommend this book if this sounds like the type of thing you can accept. If you prefer realism, and dislike ambiguity, this will probably annoy you.