The Long War

'An absorbing collaborative effort from the two giants of SF' Guardian
A generation after the events of The Long Earth, mankind has spread across the new worlds opened up by Stepping. Where Joshua and Lobsang once pioneered, now fleets of airships link the stepwise Americas with trade and culture.
Mankind is shaping the Long Earth - but in turn the Long Earth is shaping mankind ... A new 'America', called Valhalla, is emerging more than a million steps from Datum Earth, and it is growing restless . . .
Meanwhile the Long Earth is suffused by the song of the trolls, graceful hive-mind humanoids. But the trolls are beginning to react to humanity's thoughtless exploitation . . .
And a gathering multiple crisis that threatens to plunge the Long Earth into a war unlike any mankind has waged before.
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The Long War is the second in The Long Earth series.

Terry Pratchett (Author)
Terry Pratchett was the acclaimed creator of the global bestselling Discworld series, the first of which, The Colour of Magic, was published in 1983. In all, he was the author of over fifty bestselling books which have sold over 100 million copies worldwide. His novels have been widely adapted for stage and screen, and he was the winner of multiple prizes, including the Carnegie Medal. He was awarded a knighthood for services to literature in 2009, although he always wryly maintained that his greatest service to literature was to avoid writing any.
www.terrypratchettbooks.com
Stephen Baxter (Author)
Stephen Baxter is one of the UK's most acclaimed writers of science fiction and a multi-award winner. His many books include the classic Xeelee sequence, the Time's Odyssey novels (written with Arthur C. Clarke) and Time Ships, a sequel to H. G. Wells's The Time Machine, a Doctor Who novel, The Wheel of Ice, and most recently the epic, far-future novels Proxima and Ultima. He lives in Northumberland.