Masks (Star Trek: The Next Generation #7) by John Vornholt

The "U.S.S. EnterpriseTM journeys to Lorca, a beautiful world with a feudal culture where the inhabitants wear masks to show their rank and station. Captain Picard and an away team don masks of their own to begin a quest for the planet's ruler and the great Wisdom Mask that the leader traditionally wears. Their mission is to establish diplomatic relations. But shortly after transporting, Picard and his party lose contact with the ship, and Commander Riker leads a search party down to the planet to find them.

Both men, are unaware that their searches, indeed, the ship's entire mission are part of the plan of a madman, a madman who is setting the stage for a trap that will ensnare both "U.S.S. Enterprise landing parties, and leave him to seize control of the awesome Wisdom Mask as well as the planet Lorca itself.