Over Your Dead Body — Dan Wells

John and Brooke are on their own, hitchhiking from town to town as they hunt the last of the Withered through the midwest--but the Withered are hunting them back, and the FBI is close behind. With each new town, each new truck stop, each new highway, they get closer to a vicious killer who defies every principle of profiling and prediction John knows how to use, and meanwhile Brooke's fractured psyche teeters on the edge of oblivion, overwhelmed by the hundreds of thousands of dead personalities sharing her mind. She flips in and out of lucidity, manifesting new names and thoughts and memories every day, until at last the one personality pops up that John never expected and has no idea how to deal with. The last of Nobody's victims, trapped forever in the body of his last remaining friend.