Ghost-Walker (Star Trek, Book 53)

Elcidar Beta Three -- a tranquil, undisturbed planet strategically located between the Federation and the Klingon Empire. Home to the Midgwins, a race of people who throughout all time have lived in peace with their planet, and themselves. But now, times are changing. Unwilling to embrace any form of technology, the Midgwins have exhausted their world's natural resources, and stand on the brink of global famine.

When Captain Kirk and the Enterprise arrive to aid the Midgwins, they find themselves caught up in that race's struggle for survival...a struggle whose climactic battle pits them against a creature of darkness and shadow -- an entity who roams the Enterprise corridors as if it owned them -- an enemy who will not hesitate to kill to achieve its ultimate goal...