“Fatherland” by Robert Harris.
Imagine a world where Kind Edward VIII didn’t abdicate and Wallis Simpson became queen, Joe Kennedy wasn’t forced to resign because of the public outcry about his view that “Democracy is finished in England” and “[Britain isn't] fighting for democracy. That’s the bunk. She’s fighting for self-preservation.” and went on to become president of the United States, and the Germans won the war… that’s Fatherland for you. Fascinating – I really enjoyed it.