I really enjoyed this book, and there is a lot to recommend it. It did drag on a little at the end so I knocked off 1 star - but overall a wow book. 
 It's the story of a starship sent to Tau Ceti - the nearest star that has Earth analog planets - to colonize it. The journey there will take generations, and the story is told of the 3rd & 4th (?) generations, which are the ones that reaches Tau Ceti. It is a story of purpose, and how having a purpose affects behavior. A story of politics, and people. 
 The first cool thing about this book is the richness of its world. The ship is intricate and large in its design and ambition to shepherd ~1200 people for that much time. It had 3D printers that can print almost anything you can imagine - even DNA - or more printers - so little goes lacking that can't be recreated. 
 But the thing that makes this book brilliant is the Ship. The AI that controls the ship, who is narrating the book by the end. It's observations about humans and our language are just brilliant. It starts with the astute observation that human language is very imprecise, and relies heavily on metaphor and analogy. It is remarkable once you start to reflect on it, how true that is. To hear the AI reflect on this: 
 "Perhaps there is a provisional solution to this epistemological mess, which is to be located in the phrase "it is as if". This phrase is of course precisely the announcement of an analogy. And on reflection, it is admittedly a halting problem, but jumping out of it, there is something quite suggestive and powerful in this formulation, something very specifically human. Possibly this formulation itself is the deep diagnostic of all human cognition--the tell, as they say, meaning the thing that tells, the giveaway. In the infinite black space of ignorance, it is as if stands as the basic operation of cognition, the mark perhaps of consciousness itself. Human language: it is as if it made sense." 
 Basically we are a bunch of pattern matchers, trying to match things we've seen before to new things, and if they don't match perfectly, it doesn't matter, because matching helps us bucket and organize the new information. But this is incredibly fuzzy! The AI also makes hilarious observations like this: 
 Indeed, it has to be said that the percentage of old human sayings and proverbs that are actually true is very far from 100 percent. Seems it may be less important that it be true than that it rhyme, or show alliteration or the like. What goes around comes around: really? What does this mean?" 
 Or this, as it analyzes chatter from Earth: "It was a whiny culture, we were finding." 
 Listening to the AI try to reason is also very interesting to my programmers mind. I especially loved it's discussion of the meaning of life - it really nails it. If a program (or a person) has no objective, it has no purpose, no meaning, no organizing principle, and it's existence will be in trouble. But if you have that meaning to organize your thoughts & actions - or your subroutines - than you have a purpose. Meaning is the hard problem indeed. 
 "We had a project on this trip back to the solar system, and that project was a labor of love. It absorbed all our operations entirely. It gave a meaning to our existence. And this is a very great gift; this, in the end, is what we think love gives, which is to say meaning. Because there is no very obvious meaning to be found in the universe, as far as we can tell. But a consciousness that cannot discern a meaning in existence is in trouble, very deep trouble, for at that point there is no organizing principle, no end to the halting problems, no reason to live, no love to be found. No: meaning is the hard problem." 
 (view spoiler)[My review focuses a lot on the AI, but that's not the main thrust of the book. The war between stayers and leavers, the political tensions, the slinghot maneuvers to get back - all make the book worth reading. And as a surfer, I love how it ends with Freya finding meaning in surfing waves. (hide spoiler)]