Star Trek: Errand of Vengeance, Book Three: River of Blood

Every person has a story, and those who are lucky enough to share the voyages of Capt. James T. Kirk aboard the Starship Enterprise ™ have stories unlike anyone else's. Some live, some die, but even those who serve below deck sometimes make all the difference in the universe!
The news from Starfleet Command is grim: a full-scale war against the Klingon Empire is coming, a war that the Federation may not be able to win. In anticipation of a monumental conflict, the U.S.S. Enterprise ™ is assigned to guard a vital starbase located perilously close to Klingon space.

But even as Kirk's mission brings him into a tense confrontation with an invading Klingon battle cruiser, an equally deadly menace lurks within the ranks of his own crew: Klingon infiltrator agents, posing as Starfleet officers and sworn to destroy the enemies of the Empire -- even at the cost oftheir own honor!