This is Book One of the Epic Tetralogy, The Four Lords of the Diamond.
©1981 Jack L. Chalker (P)2013 Audible, Inc.
Aliens are spying on Earth from one of the four worlds that make up the Warden Diamond. But it is impossible to send agents to any of the four worlds because of unique microscopic symbionts that make it impossible for visitors to leave the Diamond.
Seeking a unique solution, each of the four worlds making up the Diamond is sent a person whose mind has been stripped of everything and who is now controlled by an agent of the Confederacy.
Lilith is the first planet to be visited. Here, Cal Tremon, stripped of his own personality, must overcome incredible odds and survive not only the incredible perils of the foreign planet but his own controllers as well.