There are 2 books in the Proxima series. The newest book is Ultima which came out in 2015.

1
Proxima

Book cover of Proxima

How would you survive on a planet that doesn't spin?

An awe-inspiring Planetary Romance from Terry Pratchett's co-author on the Long Earth Books

The very far future: The Galaxy is a drifting wreck of black holes, neutron stars, chill white dwarfs. The age of star formation is long past. Yet…
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2
Ultima

Book cover of Ultima

The mind-blowing, time-warping science fiction epic that puts grand scientific ideas into a thrilling page-turning narrative

Fresh from his latest collaboration with Terry Pratchett on the Long Earth sequence Stephen Baxter now returns to the mysteries and challanges first hinted at in his acclaimed novel PROXIMA.

In PROXIMA we discovered…