Ishmael (Star Trek, Book 23)

The U.S.S. Enterpriseâ„¢ is on a peaceful mission at Starbase 12 when a bizarre cosmic phenomenon causes a Klingon ship to suddenly vanish -- with Spock aboard for the ride. Spock's last message from the Klingon ship is cryptic and frightening. The Klingons are traveling into the past, searching for the one man who holds the key to the furure. If they can kill that man, the course of history will be changed -- and the Federation will be destroyed!