The Ganymede Takeover by Philip K. Dick and Ray Nelson

1977 2nd mass-market printing from ACE in good condition. 
 
Earth has been taken over by a strange alien force - creatures whose instinct for survival overrides any human resistance. Then a vital weapon - with the power of electronically warping the mind - falls into the hands of a terrorist group still strong enough to oppose the aliens. A weapon so powerful that it cannot be controlled. The control of Earth is in the balance - and the balance is a terrifyingly precarious one.