This is one of those books that need to be read with an already basic understanding of the author's background. First off, Kurt Vonnegut really was present at the controversial bombing of Dresden (Germany) by Allied Forces in 1945, killing thousands of civilians and Prisoners of War. Slaughterhouse 5 was the building in which Vonnegut and his colleagues sheltered from the bombing. Knowing this somehow puts a different slant on the whole story.
The first and last chapters explain all this, but in a narrative way that fits perfectly into the story - that the author was actually there, the slow progress of actually writing the book and how it was given its subtitle. Upon visiting an old friend of his from the War, the friend's wife remarked how such a book should not be written as all men Vonnegut's age had just been child soldiers, fighting in a war that was not their own. Hence, 'the children's crusade.' That's one of the major themes of Slaughterhouse 5 - that the 'men' drafted in to fight for both sides of World War Two towards the end were either too young, too old or too injured.
This is doubly true in Billy Pilgrim's case. He's definitely young, but to me at least, he seems a little... off. Mentally ill perhaps. He has an odd gait, doesn't understand basic concepts and generally needs somebody to push him along from behind. That might just be my interpretation, but that's the idea I took from it. The point is, he shouldn't have been there, along with thousands of other people unsuited for conscription.
So, the aliens -and no, I'm not joking. The timeline flits about constantly, in a Time Traveller's Wife kind of way, and it does get a little confusing at times. The concept is that Billy was abducted by aliens (the Tralfamadorians) and exhibited in a zoo on their home planet. Eventually he starts to see time as they do - as if every moment of the past, present and future is happening all the time. Time doesn't progress; it doesn't need to if you can see every second of every day at once.
While I did enjoy Slaughterhouse 5, I didn't seem to really connect to it. Maybe it was because it was more absurd than I expected, or perhaps I just wasn't in the right frame of mind. Either way, it is worth a read. Just keep in mind that Kurt Vonnegut's statistics and viewpoints aren't all strictly correct - apparently the Nazis exaggerated the death toll as part of their propaganda schemes. The author states it was around 130,000, but recent investigations show it was actually more around 25,000. Not that that's not bad enough, mind you.