This book is fantastic. It's not perfect, it's really damn far up there.
I can't even figure out where to start reviewing this, so this will be a bit rambling.
The setting: A future earth where a solar storm a century ago destroyed much of the ozone layer and subsequent global warfare has eventually nuked chunks of the world into oblivion further reducing the population, and moving the power centres far from the west and Europe. By the time earth found it's feet again, and moved out toward the stars settling the moon and near planets, someone else had moved into the outer solar system, claiming it as their own. And they are not friendly, and they are not interested in talking.
Two years ago, a ship was sent out to try to negotiate, and was blown to pieces. Now Earth is sending another ship to the border at Jupiter, a ship built to do one thing: Kill outsiders. To show we're capable of it, and not about to sit back and let the interlopers keep us cowering in fear on one damaged planet.
The worldbuilding is fantastic. Earth is a broken home, but it's still home, and it's reached the point where gender, ethnicity and religion are more or less irrelevant - but nationality, or rather citizenship is a different matter. There's a nominal global government, but it's not a happy star-trek style post-scarcity utopia, the elections are heavily fought, and the independent nations outside of it are continually causing trouble. Still, the captain of our starship, Elena Gonzales Estrella, is a Spanish woman whose mother lives on the moon. Her XO is an orphan from a refugee camp in a radioactive wasteland in northern Africa, who has no idea of his real name. Several crew members are clearly glbt (although, this is no romance, and there is no sex going on here).
The rest of the crew are equally diverse, and it's utterly refreshing to read. Elena, for example, is a controversial choice as captain for a multitude of reasons, but her gender is not remotely one of them. The problem of mentioning this in a review is, it's in your face not because this diversity is commented on or paraded, in fact Alan has done a spectacular job of just not commenting on it at all. It just is. This is how sci-fi should be written. Hell this is how every book should be written.
The plot is a little complicated, and I'm not generally a fan of split timelines and flashbacks. Most books, it would have worked just fine laying out the action chronologically. Not so here, I think starting in the present, Elena and her crew very near to the outsiders border, and then flashing back six months to show how the trip started, is not only effective, it's almost the only way the story could work.
Finally there is a religious aspect to this book, which is again, fascinating. If you're not much for religion in your sci-fi, this is religion as politics, more than as observance or philosophy. The religions mentioned that I am familiar with, I found to be respectfully handled, so I'm going out on a limb with this five star rating that the major one I'm not familiar with is also being handled as respectfully. I'd be interested to hear a Jewish perspective on this, because I'm well aware I could be wrong here.
Anyway, this book is smart, complicated and yet full of action. There's deep political manoueverings and people manipulating other people, but it's also got a giant ballista masquerading as a spaceship with a solar sail :) And I read it all in one go, because despite the layers and complexity, it's also really well written and the characters are very engaging. Highly recommended.