...for a whole host of reasons. I'll be honest, I bought this having read about it on TV Tropes, and I think the line which sold me on it was 'time traveling, SAS Prince Harry'. Well you certainly get that!
Beyond that however, this is a very well written book, and in terms of the ideas explored in across it's pages, fascinating. It does show it's age somewhat (like most books set 20 minutes into the future tbh), but even so it gives us a somewhat chilling vision of a world of 2021 as if the War on Terror had actually extended into an all up war that had rumbled on for decades (along with the consequences of such warfare on the world's militaries and the continuance of social trends of today), then goes ahead and juxtaposes that brutally with the martial and popular culture of the 1940s. Could have gone so wrong, so easily, yet it works brilliantly.
Which leads me onto the other thing about this book (and it's sequels for that matter): it gives us a very close look at the social attitudes of the 1940s and the heroes of WW2. All too often, literature (and just about every form of media) tends to look back on that time as a golden age, where for the Allies, all was noble and grand, and where the figures were genuine all-round heroes of legend, whilst for the Axis, all was oppressive and evil, and all of their soldiers and scientists and leaders were utterly inhuman monsters. This book doesn't. It shows us it all, the heroism and the racism and the sexism, the heroes, the lunatics, the geniuses, and the... well, bastards. Even more refreshingly, it does that for both the Allied and Axis powers, and doesn't pull any punches for either of them.
And yet along with all of that, it still manages to retain a sense of humour (such as that wonderful moment involving FDR, Eisenhower and a comment about how since he wasn't president yet, Eisenhower still had to work for a living), and despite the introspection, the action sequences are some of the best I've ever read.