Stephen Baxter

Thousands of years ago, a massive colony ship arrived at the Hearth - the celestial birthplace of millions of planets, ranging from habitable earth-like worlds to unimaginable hellscapes of pressure and heat. Using lightsails to navigate, humanity has spread itself across dozens of these worlds. But they have also forgotten their beginnings, where they came from... and a terrible secret is about to be unveiled.
For Commander Ulla Breen, on her first tour of duty aboard a patrol sail-ship, the universe is about to change around her. Attacked by an unknown and unthought-of enemy, she and her fellow crewmembers will face slavery, punishment and death - and so will their home planets. Because someone else has seen the richness of the inner Hearth, and plans to take it for themselves. A new enemy, but one who seems disturbingly familiar.
And perhaps knows more about the history of the Hearth than even Ulla and her crew. Faced with a complete upheaval of all she thought she knew, Ulla must survive long enough to come up with a plan - one which will unite all the disparate elements of the Hearth, and perhaps discover the reason why humanity came to Hearthspace in the first place .