I don't understand why people claim that it's science fiction. Just because there are aliens in it? Come on!
It's a book about war. Specifically, World War II; more specifically, the firebombing of Dresden.
The author uses a disturbed protagonist who believes he was abducted by aliens to justify the book's unique non-linear structure that jumps around in time. It's largely an autobiographical work, spiced up with the narrative device of Tralfamadorian aliens.
My audio version had a bit of conversation between the author and (I think) his editor at the end. That was also great.
It's worth noting that I don't usually read "realistic, historical" books about wars because I have zero interest in reading unnecessarily detailed descriptions of people being thrown into the meat grinder of suffering. This book is not like that.