I've enjoyed each Vonnegut book I've read. This is one of my favorites.
The central concept is that he was not really a Nazi (he was an American spy), but that, to some degree, it doesn't matter. It seems to ask if it matters what we really are if we are seen as something else with near universal agreement. And, is the identity others have made for us worth something to us, does it make us significant in the only way we can be? In the end, is it even possible to be something distinct from how people view you? Or, is any distinction there meaningless since it doesn't translate into anything people can see and touch and hold?