It's about human space exploration, a hard sci-fi. Possibly spoilers....but I've never really minded spoilers, so just...be careful.
It opens with a sort of a prologue, set some five hundred or so years in the future (possibly the Revelation Space universe) and they are discussing a way to commemorate a lady who helped advance space technology: Bella Lind. Then the book flashes back to the past, around 2050 or so, and traces the path of Bella. She's the captain of a comet-mining space ship, and also commander of the only ship that can chase Janus, a moon that has suddenly deviated from its orbit. On the way while they are chasing the moon, one of the people under Bella--her friend, Svetlana--discovers that the corporation that pays them has lied and sent them on a suicide mission to gather data. It is too late, though, by the time that Bella decides to act, and then a mutiny ensues, dividing the five hundred crew members down the middle. Svetlana wins, but is forced to land on Janus.
The book then follows their ensuing struggles to live on an unhospitable, often hostile and vicious moon. As Janus is traveling--under its own power--at speeds close to light, time dilation occurs, Earth signals are no longer intercepted and the protagonists leap into the future.
There's more to that, but it would really get into spoilers then.
This is a book about friendships, too, and friendships turned sour; also about how sometimes the choices are incredibly hard to make but necessary all the same. It is about people and how they change (though a couple of them were too-good-to-be-true).
What was unsettling (for me) was just the time dilation--they have travelled eighteen million years into the future, in about 100 years.
But if you like sci-fi, and don't mind that length (about 400 pages, standard Alastair Reynolds) I highly recommend you read this book. Just have a newspaper next to you when you read it, just to confirm with yourself that it is still 2009. It's very engrossing.