TOS: Ishmael by Barbara Hambly

Blurb:
The U.S.S. Enterprise™ 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!
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One of my absolute favorites among the TOS novels, and probably my favorite one of the early ones. Seeing Spock in this fish-out-of-water story is just great. The 23rd century framework isn't quite on par, but doesn't hurt the novel too much.
The U.S.S. Enterprise™ 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!
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One of my absolute favorites among the TOS novels, and probably my favorite one of the early ones. Seeing Spock in this fish-out-of-water story is just great. The 23rd century framework isn't quite on par, but doesn't hurt the novel too much.