1989, Pocket Books, New York
The Enterprise is sent to a planet called Lorca, which is inhabited by descendants of Earth who travelled there 200 years ago and subsequently forgot their technological advances. The people of Lorca all wear masks to signify their occupation and position in society. When Captain Picard's away team goes missing (when does it not?), Commander Riker heads his own team to retrieve them.
I found the Lorcan culture and history fascinating. This is more of an anthropological Trek story rather than a technological one - they seem to be the two main genres of the Trek books. The plot line was pretty basic with Prime Directive issues, technology issues, and losing all the command staff on a hostile planet issues. And Picard pulls a Kirk with the exotic alien female!
Link to journal at bookcrossing