Star Trek: The Entropy Effect: Vonda N. McIntyre

The crew of the Enterprise have been summoned to transport a dangerous criminal from Starbase Prison to a rehabilitation centre. Dr. Mordreaux is accused of promising to send people back in time, then killing them instead. When Mordreaux escapes and kills Kirk, Spock must travel back in time to avert the disaster. Mordreaux's experiements have thrown the entire universe into a deadly time warp, so Spock must not only rescue Kirk, but fight Time itself before the universe closes in on itself - the Entropy Effect. Cover art by Wayne Barlowe.