Sequels aren't easy to write, but Brennan did a wonderful job. Everything in Warrior & Witch flows out of the events of Doppelganger. The main character, Mirei, has overturned centuries of tradition and religious belief, discovered a new kind of magic, and begun a revolution in how witches and doppelgangers are raised. Unfortunately, not everyone is happy about these changes.
W&W sees the witches of Starfall turning against themselves, with Mirei racing to rescue the other doppelgangers from those who would destroy them as abominations. But not all of those doppelgangers are so eager to be rescued, or to lose their individuality. We also learn more about the witches' culture, including the nature of the mysterious Cousins. Mirei explores the limits of Void magic, as well as the duality of her newly-merged self.
One of the things that most impresses me about Brennan's writing is her honesty. She has created a fascinating world, and she refuses to shy away from some of the uglier implications of that world. In her first book, the ritual murder of (soulless) infants was a natural outgrowth of the religious beliefs and magical rules of that world. In book two, she continues to write with that same courage. Some terrible things happen, not because the author wants to get an easy, knee-jerk reaction from the readers, but because they must happen. Because the characters in this world are desperate, and it's not just the "evil" characters who sometimes cross the line. Life is rarely as simple as black and white, and Brennan does a masterful job of capturing that messy complexity. I love her characters, both for their courage and for their flaws.
There were one or two moments where I was thrown out of the book. A plot thread that seems to disappear for more than half the book before popping up again ... a bit of foreshadowing that felt a little too heavy-handed for me ... but I'm picking nits here. Brennan's first book was good. This one's even better.
As I understand it, Warner Books only contracted for two books when they signed Brennan. I certainly hope they hurry up and sign her to write more.
ETA: Unfortunately, Amazon has posted all of the Doppelganger reviews on the W&W page, so I can't post this review on Amazon. Grumble...