"Spock must die !"
Clearly no real trekkies here, or you would recall this novel (by James Blish, it was the first non-canonical story to be published, I believe).
The plot centres around a Klingon operation to neutralise the Organians, by cloaking Organia in a tachyon field. They reason (correctly as it happens) that since Organians are beings of thought, they exist as tachyons. Therefore the tachyon field will constrain them, meaning the Organian Treaty can't be enforced.
Meanwhile, far, far away, the Enterprise is safe from the war the Klingons are starting, and pondering why the Organians haven't interceded. They decide the only way would be to visit Organia. Unfortunately it's light years away, while the transporter only has range of 16,000 miles.
Luckily Scotty has an idea about modifying the transporter to use tachyons. This results in a detailed description of how the transporter currently works, with a whole slab of philosophising thrown in for good measure, to loop the narrative back to the opening chapter where McCoy is grumbling that he's is reality "dead" because the transporter has dissembled him and ressembled him, and therefore he isn't the same person.
Spock is chosen to be beamed to Organia. However when the transporter is energised, the tachyon cloak (which the Enterprise crew are as yet unaware of) reflects the beam, and a *duplicate* Spock is created. Unfortunately for the crew (but fortunately for the plot) Scotty had to shield the transporter pad with some protective material which just just happened to be opaque. So they couldn't tell who the original Spock was. It then transpires that the reflection process resulted in an "anti-Spock" whose mental functions were opposite to the real Spock, and whose motivations are to *aid* the Klingons, rather than the Federation. Of course much hilarity ensues.
Hows that for a precis of a book I last read 25 years ago. Did I leave anything out