From School Library Journal

YA-- While on a routine diplomatic mission on the planet Muscae IV, Kirk disappears, leaving Dr. McCoy acting captain. When a Klingon vessel arrives laying claim to the planet, its landing party also vanishes, resulting in a confrontation with the Enterprise. While Doctor's Orders does not boldly go where other novels haven't gone before, it is a fast-paced, well-written adventure in which the characters remain true to the Star Trek universe. Dr. McCoy, with his wry sense of humor, brings an added dimension to the story.
Copyright 1990 Reed Business Information, Inc.

From the Publisher

When Dr. McCoy grumbles once too often about the way the U.S.S. EnterpriseTM ought to be run, Captain Kirk decides to leave the doctor in command while he oversees a routine diplomatic mission. Kirk beams down to a strange planet nicknamed "Flyspeck" to negotiate its admission into the Federation, leaving Dr. McCoy to enjoy his new authority.

However, the doctor soon learns that command is a double-edged sword when Kirk disappears without a trace. Desperately trying to locate his captain, McCoy comes under pressure from Starfleet to resolve the situation immediately. Matters go from bad to worse when the Klingons arrive and stake their own claim on Flyspeck.

Then another, more deadly power threatens them all, and suddenly Dr. McCoy and the Starship Enterprise find themselves pitted against an alien fleet in a battle they have no hope of winning.