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

Somewhere, from among the four human-settled worlds of the Warden Diamond, hostile aliens were spying on Earth. But no agent could be sent to investigate and report back. All trips to the Warden worlds were one-way. There, a microscopic symbiont invaded all life forms, after which life became impossible outside the Warden system. The same organism destroyed any form of machinery, so no message could be beamed back by normal means.

That called for extraordinary means, of course. One agent was chosen, then four men were stripped of their own minds and personalities, and his was imposed upon them. Hooked up properly, he could then receive their reports, without ever leaving safe territory.

Each man was assigned one world to conquer. His mission was first to find the Overlord of that world and kill him, then to take over his link with the aliens. Of course all this must be done with no help beyond his own naked ability...