In brief: it's good. An original world, interesting protagonists, a fun story, and a satisfying conclusion.
The book jumps between two points of view. Miryo is a young witch who must kill her doppelganger before she can control her own magic. Mirage is the doppelganger, a particularly gifted warrior and Hunter. Both characters are strong, independent, determined, and likable. Naturally, Miryo's mission to kill her double soon becomes far more complicated and messier than expected, drawing them into the middle of a conspiracy with the potential to destroy them both.
The doppelganger issue originally made me stumble a little. When a witch is born, a spell is cast to help her channel her magic, and to block that channel until she's ready. The side effect is a doppelganger, which must be killed if the witch is to survive and come into her power. To me, it felt a little forced. It's a wonderful setup for the novel, but I never understood why the doppelganger was a natural side effect of the magic, for instance. And for me, the ritual murder of infants is too drastic a thing to be glossed over by saying the infants don't have souls until they're five days old, so it's not really murder.
The story gets stronger as it goes. I like the relationship between Miryo and Mirage. In many ways, they truly are the same person, and it's fun to watch Brennan explore their similarities and differences, and the connection between them. They complement one another, and the book reflects that; the part where both characters come together really does feel more complete than the earlier chapters.
Bonus points to Brennan for avoiding the typical medieval European fantasyland world. This is a world of Brennan's own creation, but its roots appear to be a bit more Eastern in origin, which is refreshing. Yes, we have swords and magic and horses and witchcraft. But while some aspects were familiar, the book doesn't feel like something I've read a thousand times before. Brennan also incorporates religion and prayer into the story, not in an evangelistic, preachy way, but as a natural part of the world and the characters. Fantasy and SF often seems to ignore or simplify religion, but Brennan presents a rich, honest faith, as well as the good and the evil that can come of it.
The book includes a short preview of the sequel, Warrior and Witch. I rarely glance at these excerpts, but in this case, I immediately read it and wanted more. Happily, Warrior and Witch will be out in less than two months. Sooner would be better, but I suppose I can wait that long.
*Of course, an author's first published book is very rarely a true first book. Goblin Quest is my fourth, and from what I've seen, I'm a bit of a quick starter...