The Separation - Christopher Priest
What if...? is, of course, the question famously asked by science fiction authors, but the same is even more true for alternate history. What if World War 2 had ended in 1941? Now, What Ifs related to WW2 are quite common, and usually come with two diametrically opposed outcomes - 1) Our Universe, where Hitler is defeated in the usual manner, hurrah for the Allies; and 2) The Other Universe, where the Nazis win and spread a reign of darkness and evil across the earth. Priest takes a more interestingly ambiguous view, where the alternate outcome is not necessarily better or worse than the actual one; finding out more about this, and how it came to happen, is the intriguing mystery at the heart of this war story.
1999, in the alternate universe. A popular historian is researching the last days of the war, and keeps coming across the name J L Sawyer, who was either a bomber pilot or a Conscientious Objector, or, incomprehensibly, both. His investigations turn up the material that makes up the bulk of the book - a series of memoirs, letters and other documents from the war, that detail the lives of twin brothers, both called J L Sawyer. The documentation is full of strange contradictions, and it soon becomes apparent to the reader that these two were part of some as-yet undisclosed flashpoint of history, where the timelines split many ways, and the brothers ended up in different legs of the Trousers of Time (© Pratchett).
The historical detail is worth reading in itself, with brilliant descriptions both of bombing raids over Germany and the experience of the Blitz, but the main attraction here is the gradually-revealed details of the new world created after the 1941 ceasefire, and the mystery of exactly what occurred to end the war so quickly. Priest bravely avoids the easy answers, and even after the final picture is revealed, there are still plenty of loose ends, contradictions and uncertainties. If you're after a linear story with a clear resolution then this may not be for you, but for any fans of parallel-universe tales or 20th century history, this comes highly recommended.