Ashes to Ashes by Jenny Han

After Rennie's death, Kat and Lillia try to put the pieces together of what happened to her. They both blame themselves. If Lillia hadn't left with Reeve... If Kat had only stayed with Rennie. . . Things could have been different. Now they will never be the same. Only Mary knows the truth about that night. About what she is. She also knows the truth about Lillia and Reeve falling in love, about Reeve being happy when all he deserves is misery, just like the misery he caused her. Now their childish attempts at revenge are a thing of the past and Mary is out for blood. Will she leave anything in her wake or will all that remain be ashes?