Andromeda Gun, by John Boyd (epub/Kindle)

G-7, emissary of an alien race of energy-beings, is sent to Earth to explore for energy sources his race needs. But when G-7 takes over the body of Johnny McCloud something happens—instead of exercising total control, G-7 is changed, involved in McCloud's personality and emotions.<P>And Johnny McCloud, the amoral gunfighter, is changed too.<P>ANDROMEDA GUN is the story of a strange symbiosis of man and alien, and of crucial events upon which the future history of Earth depends. Johnny McCloud/G-7, the symbiote, has the power to change Earth's history!