Slaughterhouse Five is a war book. Sure, aliens, flying saucers, and time travel put in brief appearances. As do possible insanity, bipolar disorders and coming of age rituals. But ultimately the book recounts the author's experiences in World War 2 and has a powerful anti-war message. 
 Vonnegut starts off by describing how he wandered around for decades trying to figure out a way to put into a book the things he saw and experienced whilst a prisoner of war in the Nazi city of Dresden when it was bombed to smithereens by the Allied forces nearing the end of WW2. 
 The Dresden bombing, using conventional weapons, killed over 130,000 innocent civilians. About 50,000 more than the amount of innocents killed in Hiroshima with nukes. The attack was concealed from the US public for many years, and when it eventually came out, it sort of fizzled out a lame death in the public sphere. It must have, because the US public has given its government its implicit approval for countless war crimes and mass murders on innocents in the subsequent decades; take Vietnam, Afghanistan and Iraq as places to start. 
 War is inevitable, says Vonnegut. There will always be a powerful government sending its youngest and brightest to die in the name of patriotism, which is an euphemism for money, power or oil depending on the war that's in progress. It plainly sickens the poor man, and the novel is written in a tone of deep, pained apathy; the phrase 'so it goes' used whenever someone dies, or something disastrous occurs, this happens a lot here. 
 The prose is simple and straightforward. And despite the pained apathy, rolls along at a cheery clip with heavy doses of dark humor. 
 The protagonist, Billy Pilgrim is kidnapped by aliens and acquires some of their ability to perceive all time at once. So he lives through his experiences in Dresden, his childhood, and adult life in successive spurts. 
 The 'Tralfamadorians' can see everything that happened, is happening and will happen as if it was depicted on a nice old tapestry slung out in front of you. Obviously this field of visions spans many dimensions. 
 I think Vonnegut is trying to say that human beings are restricted in the way we perceive reality, and consequently assume far too much about it. I'm with him 100% percent on that. But he then twists this restriction to justify a philosophy of apathetic detachment from activism, coupled with a sense of living purely for the moment, this i disagree with. 
 But all in all, this is a great book that is deceptively simple, and a fast read. Like all good speculative fiction, it transports your thoughts beyond the box.