This is the most terrifying book I have ever read. Before I read this, I'd thought Margaret Atwood's The Handmaiden was my boogeyman book. Some people are terrified of books about demons or serial killers, but none of those things can be as terrifying to me as the complete loss of personhood like in Atwood's world. 
 Butler's book takes that up to eleven, and then some. She describes spiritual violation and cultural takeover with a tone of complete inevitability. There is no sense of escape, only that you must abide or die. I think that this book would be especially terrifying to readers from conquered nations and cultures, who have had to assimilate with their oppressors and lose everything that made them who they are. The worst part is that there's no "hero side" or "villain side", the alien conquerors aren't really "bad", and the humans conquered aren't really "good". It's just complete loss and defeat to superior technology (both medical and weaponry). It's very reminiscent of certain periods in the world's history. It's very upsetting and gave me quite a few sleepless nights. I don't think I'll be able to read this again for a very long time.