The Host

The Host by Stephanie Meyer (Best Book Ever)

Melanie Stryder lives in a future society of Earth where no human is safe. A new species has envaded Earth, called Souls, who take over or "possess human minds and take their bodies. Melanie is sure that she and her brother Jamie are the last humans on Earth, until she meets Jared Howe. He is a human who hid from the souls when they took over, and she falls in love with him.  (Continue this later)

Summary:

Melanie Stryder refuses to fade away. Earth has been invaded by a species that takes over the minds of human hosts while leaving their bodies intact, and most of humanity has succumbed.

Wanderer, the invading "soul" who has been given Melanie's body, knew about the challenges of living inside a human: the overwhelming emotions, the too vivid memories. But there was one difficulty Wanderer didn't expect: the former tenant of her body refusing to relinquish possession of her mind.

Melanie fills Wanderer's thoughts with visions of the man Melanie loves - Jared, a human who still lives in hiding. Unable to separate herself from her body's desires, Wanderer yearns for a man she's never met. As outside forces make Wanderer and Melanie unwilling allies, they set off to search for the man they both love.

Featuring what may be the first love triangle involving only two bodies, The Host is a riveting and unforgettable novel that will bring a vast new readership to one of the most compelling writers of our time.