Shadow Kiss, by Richelle Mead

From Penguin USA: What if following her heart means rose could lose her best friend forever? Rose Hathaway knows it is forbidden to love another guardian. Her best friend, Lissa-the last Dragomir princess-must always come first. Unfortunately, when it comes to gorgeous Dimitri Belikov, some rules are meant to be broken... But since making her first Strigoi kills, Rose hasn`t been feeling right. Something dark has begun to grow in her mind, and ghostly shadows warn of a terrible evil drawing nearer to the Academy`s iron gates. And now that Lissa and Rose`s sworn enemy, Victor Dashkov, is on trial for his freedom, tensions in the Moroi world are higher than ever. Lying to Lissa about Dimitri is one thing, but now there`s way more than friendship at stake. The immortal undead are prowling, and they want vengeance for the lives that Rose has stolen. In a heart-stopping battle to rival her worst nightmares, Rose will have to choose between life, love, and the two people who matter most... but will her choice mean that only one can survive?