Beautiful, just so very beautiful. 
 I truly am struggling to rate this one, but it is definitely 4.5-5. 
 There is just nothing to complain about regarding this book, and just so much to praise. 
 "Envy hurt exponentially more than heartbreak because your soul was torn in two, half soaring with happiness for another person, half mired in a well of selfpity 
 and pain." 
 The world is so fascinating - a world set in the future where people in the past dabbled too much with science and experimentation, over reaching the limits of man by tweaking humans DNA to enhance them. Some people at the time were apalled at this practise and formed a separate part of society - calling themselves the Luddites. This then goes awry causing the descendants of the retrospectively named 'The Lost', were born with many limitations intelligence wise and subsequently dubbed 'The Reduced'. Now the Reduced have started occasionally producing offsprings that are free of these limitations - 'Children of the Reduced' aka CORS or 'Post-reductionists'. 
 In essence, the Luddites keep the Reduced as slaves but they do it in the name of humanity saying that they are caring for the Reduced who cannot care for themselves, but there is still the factor of slave labor to consider. The Posts, though unlimited in their intelligence, are oppressed just the same. 
 "Ancestors who had held themselves higher than God, and had been brought lower than man." 
 The plot had me enthralled, from the interactions with Kai to see what would become of them to what was going on in the North Estate, if Elliot would keep up with her experiments, who was doing what, I cared about everything that was happening, which really, really doesn't happen often. 
 In essence, but please take note that the story is about far more than this, this is the story about a young girl who fell in love with her oldest friend, he asked her to run away with him and she didn't go. Breaking her own heart. Four years later he returns, and her heart breaks every time she sees him. This is her story, but at the same time the story of their friendship, of closing the gap between castes, about whether or not they should be limited by protocols set in place that limit the entire scope of their world. It is about duty and sacrifice. 
 In their discussions of Posts and Luddites topics such as playing God, what we would do for those we love and loyalty get brought up quite often. 
 "When we have the ability to save someone's life, and we decide they aren't worthy of being saved--isn't that playing God as well? 
 The characters. Elliot is brave, kind strong and smart. She is also self-sacrificing and far too altruistic for her own good. But most of all, this girl has an indomitable will. She is strong. Her father is a tyrant, her sister is a typical spoilt rich girl. But Diana doesn't over do these stereotypes, it just is what it is. There is just a whole cast of well fleshed out support characters that made me love them and see Elliot through their eyes. 
 We don't get to truly see who Kai is in the now, but through Elliots memories and through the letters. 
 "In every letter, in every line, she saw him. He hadn't changed - he'd only grown into the man he'd meant to be." 
 "Three syllables and three thousand memories." 
 The letters.... 
 Genius. Heart wrenchingly beautiful. We see the friendship that Kai and Elliot had, and how they grew up together. How much they meant to each other and everything that happened before. This window into the past made this a beautiful book, and quite honestly it's not heavy on the romance. Their foundation is friendship and we get to see that foundation built. And I loved it. 
 "For four years she'd subsisted on memories of this--his voice, his face, the sound of his breath and his heartbeat. She felt him like a leaf feels the sun, like a magnet feels metal." 
 "I don't need to see the trail to know you're at the end of it. My grandfather's compass may not work, but mine is still true." 
 We'll pretend we're the ancient explorers, and find our way by the stars