Old book, maybe in the 70's.
A man hides in a cave and an alien creature drops on his head and they become a symbiosis. It gives him the ability to delve into disturbed minds and help them rehabilitate.
Later he uses the acquired knowledge against some bad people by feeding back the horrors his patients have shown him.
It also starts to rebuild him so he can survive situations, 2 hearts, denser structure, etc.