BEFORE making his name with Discworld, the grand work of comic fantasy, Terry Pratchett wrote two science fiction novels.

The long-gestating idea for another now finally comes to fruition with the help of Stephen Baxter.

The Long Earth is a seemingly endless series of parallel Earths to which, using a cheap home-made device, almost any human can easily “step”.

In the tradition of science fiction’s golden age, the ramifications of this one simple “what if?” provide a wealth of material, as some seek to profit from the new frontier, and others see an opportunity for escape.

Mainly, though, we follow Joshua Valiente, one of the first steppers, on an airship odyssey out into the endless Earths, in part to find out whether any of them are home to indigenous intelligent life.

Satisfying in itself, the novel nonethelessleaves a compelling setting ready for sequels which may prove even more intriguing.