Despite the generic YA cover, this isn't paint-by-numbers fantasy. This book was clearly written out of a deep appreciation for and knowledge of fantasy tropes.
Nalia is a princess whose only real joys are old documents, languages, and her one friend, Kiernan. She's shy and clumsy, but she hopes to grow up to rule her country well. Then her parents summon her to their throne room and reveal that she isn't a princess at all. 16 years ago they paid her father for her, so that the real princess could remain in hiding, safe from a prophecied death. Now that the prophecy has been avoided, the real Nalia can be brought to court, and the false Nalia--truly named Sinda--will of course have to leave. Her former parents give her a small sack of gold and send her to live with her aunt, a woman she's never met. Her aunt, a poor woman who works hard and likes being solitary, isn't cruel but is not particularly pleased to be saddled with her late brother's useless daughter. Sinda has courtly accomplishments, but was so divorced from the ordinary world that she has to learn how to dress herself.
After miserable months at her aunt's, Sinda realizes that the uncomfortable feelings she's been having are her magic, long repressed by the court magicians' spell to make her resemble the princess. She returns to the city she grew up in and begins training her magic. She resumes her friendship with Kiernan and even strikes up a tenuous relationship with princess Nalia, who is having a hard time adjusting to life in the palace. But then, just when everything seems to be settling into a good new pattern, Sinda makes a terrifying discovery. The Princess Nalia isn't the real Princess Nalia, either. The real princess is still somewhere out there!
I really liked this book. Sinda has a unique and likeable voice; I immediately related to her and wanted the best for her. And I loved the twists on prophecies about princesses--the plot is so clever, without being overcomplicated. I liked that no one (save perhaps SPOILER) is downright bad, but that entrenched power structures and privilege do a lot of damage, not all of it intended.