Diane Duane 
Doctor’s Orders [EPUB ebook] 

Dr. Mc Coy finds himself in over his head when put in command of the Starship Enterprise in this electrifying Star Trek adventure.When Dr. Mc Coy grumbles once too often about the way the Starship Enterprise ought to be run, Captain Kirk decides to leave the doctor in command while he oversees a routine diplomatic mission. But Mc Coy soon learns that command is a double-edged sword when Kirk disappears without a trace. Desperately trying to locate his captain, Mc Coy 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 the planet. And when another deadly power threatens them all, Mc Coy and the Enterprise are pitted against an alien fleet in a battle they have no hope of winning.