Revenant Gun

These three books were great, set in a strange universe of calendrical warfare. I still can't explain what that is, but that doesn't matter. Just remember Clarke's Law. What made this book extra challenging was the fact that the protagonist exists twice, and at least at the start, is referred to by the same name, no matter which one of them the story currently focuses on. Once again, it all netaly comes together in the end, and makes for a good “final” book.