Initial thoughts: I know what you're thinking: "Rose! Woman! 3-stars? 3-stars! Explain yourself this minute! You said you really liked this book, why are you giving it only 3-stars?!"I'll explain, I promise. XD First things first, I have to give it to Kat Zhang; her narrative kept me reading through this book from beginning to end, and I liked the premise of the story. For a good while, it kept me engaged in the toggle between Eva and Addie's personalities. The beginning of the novel was VERY good.I think somewhere around the middle to 60%, the momentum the story had in the beginning started to slow down, and the worldbuilding - collectively speaking - left me with a lot of questions. I also think that, despite the minimal focus on the romance (which was a GOOD thing in this book), I still felt like it wasn't quite vetted out as much as I would've liked to see.I did like "What's Left of Me" overall, I just didn't love it as much as I thought I would given some caveats. I do think that this will appeal to quite a bit of people, though.Full review:"What's Left of Me" is an interesting book to reflect upon, in my vision of things. The concept of it, by far, is one of the most intriguing premises I've come across in the past year in YA fiction. The idea of twin souls sharing one body isn't new, but the fact that they have to compete for existence where it means that one of them may potentially be erased is heartbreaking, especially if those souls "complete" each other in a sense. Addie and Eva, in their initial interactions are shown to be souls that care for each other and never want to pull apart despite sharing one body and Addie being the dominant figure. I found that bond compelling in parts of this novel as I read through it. I also found that the measure this society can recognize this phenomenon fascinating, and that they would take an active stance against erasing multiple souls to be jarring, especially with such a close bond to consider.But the question my mind tried to figure in this collective book was - why? Why did the society see the hybrids as a threat? Why were they so adamant about ridding of them? If a society is to be believed, there has to be a motivation behind the impending threat. Even if it's just to establish them as an abberation, I would've expected a little more expansion on that than what this book offered. Heck, even Dan Wells - with his work on "Partials" - showcased the divide between the societies in the world that he created. This book didn't quite show that divide, so it was difficult to see the passions/motivations behind the procedures and some of the things that Eva and Addie observed.I did admire, however, the way Zhang creates the narrative in how Addie and Eva worked against their constrictions. Granted, Addie wasn't always happy about Eva trying to exert control, and the two argued contentiously about the push for dominance and working against the norm, but neither one of them wanted the other to face erasure. That to me was probably the most compelling part of the novel, in addition to some jarring betrayals and chase scenes where those challenging the status quo are taken to be "corrected".I think there are points of the novel that are hit and miss. The very beginning of the novel has fluid progression and exchanges that show the impending threat to the girls. Yet, about midway through the novel - I'll admit there were parts that dragged on a bit more than they should've for the subject matter. The push toward the end is where the novel's momentum picked up speed again, with some creative leaps that I wish had more expansion - like the nature of the operations, the establishment they were in, among other factors. The worldbuilding in this is very light, so for me it was harder to delve into than I would've otherwise. It left me with many questions as to how certain things came to be and if it were just a bit fuller, I think I would've ranked this higher.There was also the matter of the romance in this book - I'm actually happy it takes a backseat to developing the tensions and connection between Addie and Eva, but I still felt that the element was so light that it lacked enough development to pull me in - it had me at a bit of arms length as I read it. I felt the characters cared for each other, certainly, but not the pull where that tie was substantiated enough to engross or enthrall me.I think younger teens and those that are looking for a light dystopian novel with interesting elements would likely enjoy "What's Left of Me", because the idea of it is well thought, and this novel has its moments of harrowing conflict. I think, however, with a little more development, this could've been a fuller experience than what was. Still good, but not to the level that I think some further focus would've brought to the table. I'm certainly invested enough in the book to see what happens to Addie and Eva from here on, and its an interesting start to this series.Overall: 3/5Note: I received this as an ARC from Edelweiss, from the publisher HarperCollins/HarperTeen.