Classic SF unavoidable

It is a book that I keep for years in my library to the radius "must" SF. I reread it recently; it has kept its appeal, despite the wave of technology today that could make obsolete nicely. And because, here, the "machine" is only the backdrop for an essentially human dream: communication between intelligent but fundamentally different entities.
This is the story of a scientist somewhere ill at ease, the bitter memories of a lost love are forged, against his will, his passion for a strange planet whose study, after many years craze without results, the scientists began to tire. All along, we feel that not enough to exceed the cap.