Solaris by Stanislaw Lem
Trade paperback format.
Who's testing whom? When Kris Kelvin arrives at the planet Solaris to study the ocean that covers its surface, he is forced to confront a painful, hitherto unconscious memory embodied in the living physical likeness of a long-dead lover. Others examining the planet, Kelvin learns, are plagued with their own repressed and newly corporeal memories. Scientists speculate that the Solaris ocean may be a massive brain that creates these incarnate memories, its purpose in doing so unknown.
The first of Lem's novels to be published in America and now considered a classic, SOLARIS raises a question: Can we truly understand the universe around us without first understanding what lies within?