Crown of Blood — Denise Grover Swank

The third book in the Book of Sindal series, Celeste's story.
Since the tender age of four, Celeste Whelan has known her duty -- use her powers to defeat the Dark Force, a group of mages who are intent on overthrowing the Valerian government. Her erratic expression magic and her inability to master it give her the perfect excuse to seek help from the Dark Set. It also makes her the perfect martyr.
 
What Celeste doesn't expect is for the Dark Set to set her up with another expression mage, Zane Chambers. He wants nothing to do with Celeste, and the feeling is mutual. But there's no denying Zane he has answers about her magic, answers that will help her steal the magic abilities from all the members of the Dark Set.
 
But the Book of Sindal -- the book of evil spells the Whelan family has guarded for centuries -- is offering Celeste the Crown of Blood, an offer that is becoming harder to resist. Celeste has a mission to fulfill, and she'll do anything to do it.
 
Even if it means giving up everything she loves.