It's a short novella, but there are so many reasons to like it. First, we get to see the queen from her people's POV. We experience how far she is willing to go to create her army, and what she does to those selected.
Then we get Z's coming of age story. It's rather dark and bleak watching him fight for his humanity when they're literally turning him into something else, but it's also fascinating. You can't help but like and sympathize with him, especially when he seems like one of the only ones who is bothered by what the queen did to them.
Anyway, you need to read this book. It's an excellent compliment to Cinder and Scarlet, and we get to see the origin of "the big, bad wolf." It's absolutely4 stars for me. If you love the Lunar Chronicles, you have to pick this up, and if you're just thinking about reading this series, this is a great introduction (though it's more a lead in for Scarlet than Cinder).