Keith Laumer 
Plague of Demons 

THE DEMONS HAD TO STOP JOHN BRAVAISHis secret assignment was simply – to save mankind from the savage dog-like 'things’ that used their hands like men. Yet an unknown number of apparently 'human’ beings were against him too.First transformed by surgery into a superman, John Bravais probes ever more deeply into the secret nightmare world of the 'things’. At last, when only his mind remains – trapped in a vast robot war machine on the moon – only by an immense act of will-power can he give humanity a future.