This may be insane, but this book gave me nightmares. This is entirely out of scale with the tone of this thing, which is one of those 70s cheeky monkey deals about genre and whatnot. But. I have some serious issues with threats to autonomy, the kind one can find in Through the Looking Glass
and similar, so, nightmares ensued.
The one I remember best is about a giant blue square coming to kill me. The dream happened when I was a teenager, and living at home. The square could make me change my mind, like the monster in this book. I was accompanied by my twin sister, who was nothing like me, a ravel-haired Victorian type. When we moved, we moved in the same way. Holy Jeezus. When my mother finally shook me awake - I was screaming - I realized that the giant blue square was the blue of the sky out my window. I was sleeping with my eyes open.