From the back cover:
"Your sleek cloudskate skims across the Skysea. It is midnight. All day long the glass sea soaked up the scorching desert sunlight. Now, that heat lifts your ship a few inches off the glass and fills its sails. The only sound you hear is the soft swish of the diamond-edged rudder as it cuts a path across the Skysea.
One thousand years ago, the wizard Martek knew that you would come to find his sphere of power. Now, one of his glowing Star Gems shows you the way. The starlight, reflected in the glass benearth you, flickers peacefully.
Suddenly, the Skysea before you bursts up into a thousand shards of splintered glass, showering into the moonlight! A horrible creature is silhouetted against the moon. You strain against the rudder to keep your ship upright.
Too late! Your cloudskate tips onto one runner, and then tumbles over, skidding to a stop on the glass. As your companions struggle to right the ship, you turn to face the monster. You must hold the creature back to give them time. Without the ship, none of you can get off the glass before the sun rises in the morning.
Martek's prophecy spoke of heroes, tests and dangers. Are you the heroes? What are the tests? What dangers and riches lie ahead?"