(This review was originally posted at My Library in the Making.)Actual rating: 3.5I don't know if I mentioned this before, but I've only read a few dystopian/post-apocalyptic books - considering it's been the current trend - and Gravity certainly makes a nice addition to my list.In Ari Alexander's world, humans owed their lives to Ancients - aliens who, apparently, had been on Earth for a long time. After the damage that had been brought by World War IV, these Ancients formed a treaty with the humans, and under it, the Ancients would help sustain life on Earth in exchange of a future coexistence because they were having problems with their own planet. However, the Ancients couldn't survive on Earth without the Taking, a process where they get antibodies from humans.Ari was the daughter of the Commander - practically the second most powerful man in Sydia - and since position was hereditary for the higher-ups in their society, her life had already been planned out for her the moment she'd been born. She was to train physically and mentally, learn the responsibilities she would one day have, and even marry the President's son who was one of her best friends. She didn't love that last bit, but she knew she had to do it. She became what everyone had expected her of being, until she discovered something: Jackson Locke, her biggest rival in Operative training, was the Ancient assigned to her for the Taking.I love it when females kick ass, so of course I loved Ari. She was like Tris Prior from Divergent, only better at fighting. She was also intelligent and kind-hearted, and it was really fun to see her breaking the rules that she'd been following her whole life. And, like Ari, the whole time, I was also confused as to whose side I should take: the overconfident humans' or the vicious aliens'. Jackson, on the other hand, was mysterious all throughout the story. His identity, plans, and motives were always questionable, and so he was always interesting; quite swoon-worthy, too.Their romance blossomed faster than I would have expected, though. I mean, sure, Ari found out that Jackson's the Ancient assigned to her and she began working with him in his mission, but to like him in a few days when she'd disliked him for years?But I did love the world-building. Melissa West wrote a very believable future world: technologically-advanced yet suffering because of humanity's actions. And the aliens! I already believe in aliens, and add to that a Discovery Channel documentary I watched last year about aliens that have been on Earth since prehistoric times, and even the aliens in this book were creepily believable.And the pace was amazing. I was immediately thrown into action when I started reading this, and I forced myself to put it down on the first "calm" part I got to. Also, the writing was solid and straight-to-the-point.I love plots that revolve around high stakes - and I can't think of anything higher than planet domination and racial safety - but the ending was a letdown for me. I felt like it ended around twenty pages before it should have. West could've answered the most immediate questions and made me anxious for the sequel, but now I only feel like something's missing.MY FAVORITE PARTS were the very entertaining Law/Jackson scenes.