It is one of few books I actually finished last year. I only read a dozen or so full books in all.

Anyway, as for the story it's a piece of near future sci-fi. Sometime in the near future a schematic is published online which allows one to build a simple device which can be used to 'step' onto a parallel earth. This as you can imagine causes some initial chaos and wonderment but eventually 'stepping' becomes a sort of standard thing in life. People start heading en-masse to the seemingly endless string of parallel worlds all of which have slight differences to our Earth in terms of native flora and fauna, geography and climate etc.

The book has several plot threads involving exploring these Earths including the question of how many there truly are, whether it is infinite or not, the right to govern and inhabit these worlds and how mass emigration effects those who are left behind, as there are a small minority who are unable to 'step' unless carried across.

The book also questions what it means to be alive, or to be sentient. One of the characters is a robot who claims to be a reincarnated Buddhist but may just be a super-advanced AI.

All in all it is a really interesting book, and is quite left-field of anything I've ever read by Pratchett. It is lacking in a lot of his stylisms which makes me think the bulk of the book was maybe written by Baxter, an author with whom I am otherwise unfamiliar, with the pair of them contributing to the plot and the concepts explored in the book.

There. I just wrote much much more than I initially intended, which is an indicator of how interesting I found the book, that I found myself unable to limit what I had to say.