Incarnation, book 1: The Alchemy of Forever
Avery Williams2012

After spending six hundred years on Earth, Seraphina Ames has seen it all. Eternal life provides her with the world's riches but at a very high price: innocent lives. Centuries ago, her boyfriend, Cyrus, discovered a method of alchemy that allows them to take the bodies of other humans from jumping from one vessel to the next, ending the human's life in the process. No longer able to bear the guilt of what she's done, Sera escapes from Cyrus and vows to never kill again.

Then sixteen-year old Kailey Morgan gets into a horrific car accident right in front of her, and Sera accidentally takes over her body while trying to save her. For the first time, Sera finds herself enjoying the life of the person she's inhabiting--and falling in love with the boy who lives next door. But Cyrus will stop at nothing until she's his again, and every moment she stays, she's putting herself and the people she's grown to care about in danger. Will Sera have to give up the one thing that's eluded her for centuries: true love?