The Entropy Effect (Star Trek: The Original Series, #2)

The Starship Enterprise™ is summoned to transport a dangerous criminal to rehabilitation: the brilliant physicist, Dr. Georges Mordreaux, who is accused of promising to send people back in time, then killing them instead. But when a crazed Mordreaux escapes, he inexplicably bursts onto the bridge and murders Captain Kirk before the crew's eyes.

Now Spock must journey back in time to avert the disaster "before" it occurs. But more is at stake than Kirk's life. Mordreaux's experiments have thrown the universe into chaos, and Spock is fighting time itself to keep the very fabric of reality from unraveling.