Poseidon's wake - Alastair Reynolds

MANKIND HAS REACHED THE STARS.
Two hundred years after the fall of Mechanism, human society has achieved a kind of stability. There are colonies beneath the oceans, throughout the solar system, and beyond: on extrasolar planets. Vast hemi-relativistic ships connect these colonies, travelling at half the speed of light. Or rather they would, if the ominous presence of the alien Watchkeepers had not led to an enforced moratorium on interstellar travel.

But when a seemingly impossible radio signal reaches the colony Crucible, everything changes:

SEND NDEGE

It's origin is unpopulated, unexplored space. No one could be there - at least, not if they travelled using human technology -
so who could have sent it? How did they get there? And what use do they have for the disgraced scientist Ndege Akinya?

Finding the answers will require one of the greatest expeditions humankind has ever launched, a journey further than ever attempted before, conducted under the implacable scrutiny of the Watchkeepers.

But as a mission is prepared on Crucible, it turns out they weren't the only ones to see the message - or its potential . . .

'Reynolds' future is so brilliantly extrapolated . . . original ideas fizzing off every page' The Guardian
'Brilliant, self-assured, colourful space opera' The Sun

Completing the informal trilogy which began with Blue Remembered Earth and On the Steel Breeze, this is a powerful and effective story.