The concept of two souls born into every body is intriguing. Add to that a government that actively (you have NO IDEA how actively until you finish reading!) seeks to make sure all kids have "settled" (the recessive soul fades away into nothing) by the time they're ten or else they're labeled hybrids and institutionalized, and you have a delicious recipe for disaster when two souls refuse to settle. Toss in a large dose of government propoganda against hybrids and a nice big pack of lies and secrets, and the stakes are even higher.
But all of that glitzy concept aside, this book is wonderful because the writing is wonderful. I had no trouble keeping up with which soul/character was speaking as they each have such distinct personalities. I enjoyed the fact that Addie and Eva make many mistakes or panic and freeze when they should probably run because that's real. Most teenage girls with a huge secret to hide who land on the radar of a terrifying government agency wouldn't have clue one in how to deal with it. I appreciated the emotional journey the characters took as well. It was fraught with drama and mistakes and love.
I was utterly compelled. I thought about the story when I had to put the book aside to deal with real life, which is the mark of a really great story to me. I highly recommend and will eagerly anticipate the sequel!