Seelen by Stephenie Meyer

In a future where parasitic alien “Souls” take over human bodies to survive, a Soul implanted into the body of a young woman finds that her host’s memories and emotions refuse to vanish; as they struggle for control and understanding, the two consciousnesses form an uneasy alliance, join a small band of human survivors hiding in the desert, and confront questions of identity, loyalty, love, and what it truly means to belong, forcing both species to reconsider the possibility of coexistence.