Lilith: A Snake in the Grass - The Four Lords of the Diamond, #1

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 becuase 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. This is the first book of the Four Lords of the Diamond series. The others are:Cerberus: A Wolf in the FoldCharon: A Dragon at the GateMedusa: A Tiger by the Tail