Kenneth Bulmer 
Patient Dark [EPUB ebook] 

For something like two hundred and fifty years Earth had been dominated by humanoid aliens from the star world of Alishang. But man’s spirit refused to be conquered. There was a world-wide underground planning for the day of final liberation. And there were four leaders who knew the secret that would guarantee victory — the secret of ZI.Rupert Clinton, intelligence man for this underground, was not one of those four; yet somewhere deep in the recesses of his subconscious mind, he knew ZI’s secret.