The Empire of Man is a vast multi-planetary network of human colonies that have been unified by a benevolent imperial administration. But no intelligent alien life had ever been discovered. Until now, when a strange spaceship suddenly appears on the outskirts of a moderately populated planetary system. Scientists are dispatched to communicate with the aliens and soon an expedition is got up to retrace the path of the alien craft through the wormhole from which it came.
When they get to the alien system they find an amazing and extremely ancient culture the scale and technology of which is 1000s of years beyond humanity. However this species has been trapped in their lone star system for failure to successfully navigate the unique but extreme gravitational and physical geometry of their system. They had up to now never had a successful return of a ship which traversed the wormhole.
The explorers are treated to a politically manicured tour of the aliens' amazing planet. But they soon realize the full extent of the threat the aliens pose to humanity should they ever escape their system. Humanity would be overrun by the aliens in no time. They escape back through the wormhole and put out the warning.
Sorry for all the spoilers of such a great book, but hopefully someone else knows the title!
I just remember there was a cover review quote by Robert A. Heinlein, "Possibly the best science fiction book ever written."