Down to Earth
P. Wrightson
Hardcover
(Harcourt, Jan. 15, 1965)
His name was Martin, and he claimed to be a visitor from another planet. Of course, even Cathy and George didn't believe him at first, but he did have an odd way of staring at you and then there was his habit of squeezing into very small places, his occasional green glow and they way he bounced.