Keith Laumer 
Shape Changer 

Lafayette O’Leary, to his acute discomfiture, has an exhilarating and terrifying tendency to slip continua. And what might that mean?It might mean finding oneself a gypsy with a ring in his ear at one moment, and then suddenly a crippled birdman unable to leave his nest because of the awkward loss of teleporting talent.It might mean battling one’s way back to the time stream where he started – only to find himself already there, lording it over the populace like a tyrant.