It's been almost 60 years since Step Day, when the seemingly infinite worlds of the Long Earth first opened to humanity. These pristine planets, accessed by stepping "East" or "West" away from the original Earth, promised a fresh start for the human race. A young Joshua Valienté and his companion Lobsang, an artificial intelligence, were among the first to trek across the interdimensional wilds after Step Day. Their adventures in The Long Earth (2012) set the course of the Long Earth series, of which The Long Cosmos is the fifth and final entry.
Now nearly 70, Joshua has seen the pioneering promise of the Long Earth give way to mixed results. The human diaspora has led the species in strange new directions, both socially and biologically, over the course of The Long War, The Long Mars and The Long Utopia. The Long Cosmos begins with a message broadcast over these many worlds: "Join us." This mysterious invitation spurs Joshua and a cast of other familiar faces into action, leading to another step in human evolution as significant as the original Step Day.
The Long Cosmos is a triumphant high note for a series with some slow entries (The Long War and The Long Mars in particular). It's also likely the last book with contributions from Terry Pratchett, who died in March 2015. Pratchett and Stephen Baxter's final collaboration is accessible to newcomers, though perhaps best enjoyed by those who have read at least The Long Earth. Cosmos captures the same sense of wonder and possibility that made that first novel so intriguing. --Tobias Mutter, freelance reviewer