The first part of this, I loved completely, solidly 5- star if not six. Mysterious and wonderful and confusing and compelling. 
 Then it's afflicted with Wall-E syndrome: more characters show up. 
 It's still great, and mysterious and compelling. I stayed up until 2 am turning pages. Still an excellent book, but it doesn't live up to the promise of that alien, lonely Part 1. The extra characters are there for Seth's convenience and to help him (he suspects as much, anyway); he imagines animals and villains into being. 
 But that sense of not-knowing still hangs over the whole book. No matter how much Seth remembers and begins to understand, there's more unexplained. It's ambiguous and confusing in all the best ways. There's so little sense of what's real and what's construct, and Seth's uncertainty is palpable. 
 Not a book I'm likely to shake from my consciousness any time soon.