Bloodlines by Richelle Mead
I wasn't free of my past, not yet.
Sydney's blood is special. That's because she's an alchemist - one of a group of humans who dabble in magic and serve to bridge the worlds of humans and vampires. They protect vampire secrets - and human lives. But the last encounter Sydney had with vampires got her in deep trouble with the other alchemists. And now with her allegiances in question, her future is on the line.
When Sydney is torn from her bed in the middle of the night, at first she thinks she's still being punished for her complicated alliance with dhampir Rose Hathaway. But what unfolds is far worse. Jill Dragomir - the sister of Moroi Queen Lissa Dragomir - is in mortal danger, and the Moroi must send her into hiding. To avoid a civil war, Sydney is called upon to act as Jill's guardian and protector, posing as her roommate in the unlikeliest of places: a human boarding school in Palm Springs, California. The last thing Sydney wants is to be accused of sympathizing with vampires. And now she has to live with one.
The Moroi court believe Jill and Sydney will be safe at Amberwood Prep, but threats, distractions, and forbidden romance lurk both outside - and within - the school grounds. Now that they're in hiding, the drama is only just beginning.