Stephen Baxter & Terry Pratchett 
The Long War 
(Long Earth 2)

»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.