The Willing by Lyn Hemphill

Kitty Wilson doesn't know how, but she can bring people back from the dead as long as someone is willing to take their place and keep the universe in balance. Oxford student Talia McGregor's life goes from bad to worse when she's killed in a hit-and-run. When the drunk driver responsible agrees to sacrifice his life to save Talia's, Kitty makes the trade and brings Talia back to life. Neither of them expects the ghost of Matt, Kitty's BFF, to come back with her. Kitty has to figure out how to untether Matt's ghost from Talia and bring him back for good. In her quest to discover the extent of her magic, Kitty must confront a secret society with questionable motives and connections to tragedies in her past, and a magical bloodline she never knew was hers. As the stakes and the spells rise, Kitty and Talia rely on each other to survive, but can they open their hearts and trust the magic of love?