I've heard that this is perhaps the best book Arthur Clarke has ever written. I also understand that the rest of this "Rama" series isn't nearly as good.
from the publisher:
At first, only a few things are known about the celestial object that astronomers dub Rama. It is huge, weighing more than ten trillion tons. And it is hurtling through the solar system at inconceivable speed. Then a space probe confirms the unthinkable: Rama is no natural object. It is, incredibly, an interstellar spacecraft. Space explorers and planet-bound scientists alike prepare for mankind's first encounter with alien intelligence. It will kindle their wildest dreams... and fan their darkest fears. For no one knows who the Ramans are or why they have come. And now the moment of rendezvous awaits -- just behind a Raman airlock door.
"Mr. Clarke is splendid...We experience that chilling touch of the alien, the not-quite-knowable, that distinguishes Science Fiction at its most technically imaginative." -- The New York Times [an error occurred while processing this directive]