What's Left of Me is set in an alternate reality where every body is born with two souls. One soul is dominant, and by the time the body is a teenager, the other soul is supposed to naturally fade away. But it doesn't always work that way - not for Eva, at least, who is still most definitely alive in the body her sister, Addie, controls. Addie has to keep Eva's existence a secret, because "hybrids" are considered against nature and are taken away by the government. Then everything changes when the girls discover a pair of siblings who are both hybrids, and learn that Eva could potentially regain control... and that perhaps being one-souled isn't so "natural" after all...I first discovered this book when all of my Goodreads buddies started raving about it. I was intrigued and thought the plot sounded ambitious and interesting, so I was keen to give it a go. Now I find myself in a strange position - I loved the book for the characters, writing and pace, but am left with so many questions and frustrations with the world building, I just don't know how to rate it. I think the plot was perhaps a little too ambitious. It's supposed to be an alternate reality where everyone has always been born with two souls, but if this were the case I feel like society would have developed in a very different way to our own. Relationships, families, schooling, work - all would look different if every body had two personalities in it. But it doesn't. Aside from the mildly dystopian creepy government control/conspiracy thing, the society in What's Left of Me looks exactly like ours. Granted, everyone is supposed to have one soul by adulthood in Eva's culture, but this hasn't always been the case, and surely society would have developed differently in some way. It just didn't make any sense.It also didn't make sense that everyone would just pretend like the second soul never existed. If both souls are treated equally in their early childhood, and both loved by their parents, there would be considerable grief over losing one of those souls, not just for the one left behind but for the family, too. Even if it's the "natural" way of things - especially if it's the "natural" way of things - surely there would be some sort of cultural practice in place for this kind if thing. A type of funeral or memorial to honour the lost. To just pretend like they didn't happen, after loving them for years? That's just not logical, and it bothered me.But. Those gripes aside, I really enjoyed What's Left of Me. Eva was a great character, and I really connected with her frustration and also her burning desire to live and not just exist. It was a smart choice to have the secondary soul as the protagonist rather than the dominant one, as it definitely made for more compelling storytelling. The relationship between Eva and Addie was complicated and fascinating, and Zhang did a wonderful job of exploring the tension that their unique problem created. The secondary characters were also well-written, and I especially liked the blossoming romance between Eva and Ryan - including, oddly enough, the way it wasn't central to the plot. Just a nice part of it. I'm so over YA books where the plot functions as nothing more than a vehicle for a swoony boy, so it's nice when the love story forms part of the plot, and doesn't completely obliterate it.There's a lot of action in What's Left of Me, making it difficult to put down. I love it when a book stays on your mind even when you haven't got it in your hand. It's been awhile since I experienced that, so I got really excited while reading this. I was also grateful for the sense of closure at the end of the novel. Don't get me wrong, it's part of a series and there's definitely a lot of open threads and avenues to be explored in the sequel(s), but it didn't just end abruptly and frustratingly as so many other first instalments seem to do. It's not a massive cliffhanger, and impatient little ol' me loved that. In fact, despite my misgivings, I enjoyed What's Left of Me so much they I found myself still thinking about it days later. Sure, I wish the world building was stronger, but I can't deny that it had an impact on me, and for that I guess I have to rate it highly. I still have a lot of problems and questions that I suspect won't be answered in coming instalments, but I loved it nonetheless, and can't wait to see what Zhang has in store for us next.I received a review copy of this book from the publisher via Netgalley.This review also appears on my blog.