Solip:System (#1.5)

They call it Black Mind. Using this covert technology, Reno has written his own consciousness over that of Albrecht Roon, one of Earth’s greatest enemies. A saboteur surrounded by enemies, he must act quickly, and without giving himself away, in order to turn the Orbital oppressors against each other and bring down their entire system.  

He’s living in a labyrinth of paranoia, surrounded by bodyguards and treacherous rivals. And then he discovers that Black Mind is not a complete success—Roon still lives inside him, and Roon is mad.