Axis

This is a great follow-up to Spin: it covers a shorter time span, less breadth of the Spin Universe, but goes into depth about what the Hypotheticals are, and how they interact with the universe - and with humans.

Because that's the centre of the story: the boy who is made to interact with Hypotheticals, and ends up doing so - spectacularly.

I liked this book a lot. I am now going to binge on Number 3.