Fredric Brown was one of science fiction's great authors, and this in my opinion is his finest novel, albeit overlooked by most critics and in fact out of print for years. I treasure my copy and if you can find it, buy it!
This is the story of a being from a distant star who is exiled from his/its home planet to a randomly selected primitive world: the Earth. He is sent here by teleportation. Under the laws of his exile, if he can return to his home world, he is both forgiven his crimes, and becomes a hero.
The being (The Mind Thing) has no arms or legs, and looks much like a tortoise shell minus the head and legs. But he possesses a terrible power. The Mind Thing can enter the mind (and thus control the body) of any sleeping creature. The catch is, it can only exit the possession of such a creature by killing it, i.e. by causing it to commit suicide.
The story focuses on the efforts of The Mind Thing to control the necessary humans to enable it to return to its home world. Oh, and by the way, The Mind Thing has determined that human beings are ideal hosts for its race, so if it succeeds in returning, eventually its race will raid the earth for its slave potential. More would be telling, but this is a wonderful, imaginative, and chilling tale that nevertheless is not without the usual whimsical humor and irony that the great Fredric Brown always imparted to his stories.
Don't miss this one if you can find it!