Werewolf Principle — Clifford D. Simak

His body hosting a pair of strange alien presences, an amnesiac space traveler returns home to an unrecognizable Earth

Many centuries in the future, a two-hundred-year-old man is discovered hibernating in a space capsule orbiting a distant star. Transported back to his home planet, Andrew Blake awakens to an Earth he does not recognize -- a world of flying cars and sentient floating houses -- with no memory whatsoever of his history or purpose. But he has not returned alone. The last survivor of a radical experiment abandoned more than a century earlier, Blake was genetically altered to be able to adapt to extreme alien environments, and now he can sense other presences inhabiting his mind and body. One is a biological computer of astonishing power; the other is a powerful creature akin to a large wolf. And Blake is definitely not the one in control. With his sanity hanging in the balance, Blake's only option is to set out in frantic pursuit of his past, the truth, his destiny -- and quite possibly the fate of humankind.
 
A bravura demonstration of unparalleled imagination, intelligence, and heart, The Werewolf Principle addresses weighty issues of genetic manipulation that are as relevant today as when the novel first appeared in print. One of the all-time best and brightest in speculative fiction, Grand Master Clifford D. Simak offers a moving, stunning, witty, and thought-provoking exploration of what it means to be human.