It's strange. This book doesn't feel like a *book.* It starts in a place that won't entirely make sense if you haven't read 1 and 2. It has a somewhat satisfying ending, but at the same time feels a little bit like the action is just being dragged out for the purpose of having a fourth book. It does pass, just barely, my criteria for a book to have its own arc which works independent of the rest of the series. But from a purely technical standpoint, I found this book to be kind of disappointing.
Luckily, from a content standpoint, it managed to make me overlook the issue. As far as bringing the stakes and adventure, this one was probably the best of the series. From a "would I reread this" standpoint it's far inferior to the first book. But the newly introduced characters are fantastic, the action was solid and perhaps the best thing of all, it's made it difficult for me to think about how long I have to wait for the next (and honestly, hopefully last) book in the series.
The titular Cress is a young woman who has a preternatural talent with computers. She's been longed up on a satellite, basically alone, doing mission after mission in an attempt to get some sort of love and recognition out of the only person who ever visits her. But after being contacted by Cinder and her friends at the end of the last book, she's now facing a wonderful, scary prospect - getting rescued and interacting with real people, not just the ones she watches on the computer and her clinical and scary "mistress."
But it's a novel, and things have to go wrong. And go wrong they do, in spectacular fashion, as characters are split apart and forced to work against odds they never expected, all with one goal - to get back together and stop the impending wedding of Emperor Kai and Queen Levana. Sure, maybe part of it is that Cinder still harbors feelings for Kai. But mostly, they just can't let her get a royal foothold on Earth.
Cress was the star of this book in all possible ways. In a series full of hard-edged characters ground down by their experiences, she is a breath of fresh air. Her joy at finally getting to Earth, her romantic visions of what people and places are like (thanks to growing up learning about life from movies and TV) and her willingness to help even as she's not really much use in a fight - she brought a wonderful new dimension to the story. I couldn't shake the mental image of this Rapunzel as the one from Tangled. Which is pretty awesome.
Overall, if you're into the series, pick this one up. It's definitely a strong book, much stronger than the second in my opinion, and it's just a lot of fun. But if you're new... absolutely, under no circumstances, start here. There's just too much you need to know going in.