I'm really surprised at many of the other reviews I've read here, wondering what caused Ben to be the 'monster' he is, etc. and asking what the point is.
The whole time I read this book, I was thinking:
1) I related to some of Harriet's frustrations and unfairness toward the baby consuming all her energy while in her stomach - I thought, this doesn't have to be a monster; it could easily be any pregnancy at all, especially if it's her fifth one in six years!
2) Ben sounded very autistic. I have studied the subject quite a lot, I have been diagnosed with autism myself, and I know people with deeply autistic children in the family who have a habit of lashing out violently, not understanding human emotions, wo mimic others' ways of behaving in order to try to be what everyone else expects them to be, and who can't respond with love, etc. I spent this whole book thinking it was such a classic portrayal of this state of mind, and feeling absolutely horrified at the way people seem to have treated such children back at the time this book was written (the hospital scene).
3) The back of the book is misleading - I kept expecting this book to turn out to be 'The Omen' or 'Rosemary's Baby', but it isn't. It plays off that sort of story from the '70s and twists it into something else - the view society has of such children as 'monsters' when in fact all they need is love, just like anyone else. The ending to the book was heartbreaking in its understatedness.
I think this is such an important book...but only if you understand what she's actually saying. It is a horror novel, but the horror is really over our lack of understanding of children who don't fit our idea of what children 'should' be.