In 1968, James Blish wrote in the introduction to Star Trek 3 that he would be writing an original Star Trek novel, the popularity of the television series warranting such an effort. By the time that book was published, in April 1969, Star Trek had already been cancelled, with only its final episode, Turnabout Intruder, yet to air. Finally, in February 1970, Bantam published Blish's (sole) original Trek novel: Spock Must Die!
The Enterprise is mapping out a region of space near the border of the Klingon Empire, a highly routine operation. Kirk comes upon McCoy and Scotty in the rec room, having a philosophical debate: is what comes out of the other end of the transporter the same person that went into it--does the soul make the jump? If not, McCoy says, then "every time we put a man through the transporter for the first time, we commit murder."
There's no time to pursue this discussion further, for Spock interrupts to announce that the captain is needed on the bridge: the Klingons are at war with the Federation, and the Enterprise is now behind enemy lines.
It should be impossible for the Klingons to break the peace treaty, but Organia (the home of the beings that instituted the treaty in Errand of Mercy) seems to have been destroyed, so they are unhindered--and having great success. Unable to get into contact with Starfleet Command without giving themselves away, Kirk decides to head for Organia, a journey of several months, in hopes that the Organians have not been destroyed, and might be able to stop the war.
Inspired by his conversation with McCoy, Scotty invents a new kind of transporter which will make a temporary duplicate of a person and sent them away at great speeds and over enormous distances, with which he proposes to transport someone directly to Organia to investigate. Spock volunteers, and so the experiment begins.
Unfortunately, things do not go as planned. After the attempted transport, they find two Spocks on the transporter pad, each claiming to be the original, and each apparently identical. The balance of the book concerns the efforts to distinguish which Spock is the duplicate (and must therefore be destroyed) and to reach Organia undetected.
Spock Must Die! is similar in style to Blish's adaptations, writ large. Blish liked to slip literary references into those short stories, and the novel takes it further, with references to works as varied as Othello, The Chronicles of Narnia, and Finnegans Wake (the last of which forms a substantial plot point).
Blish does take advantage of the freedom afforded by the increased length. The philosophical debate outlined above is not the only one in the novel, and if they are perhaps a bit overwrought, they are welcome reminders that science fiction can be about ideas, and not merely action set in space. The general increase in detail is beneficial, as well; the adaptations often seem too brief.
Sadly, I found the book's ending unsatisfying; the resolution of the plot is too straightforward. The Enterprise set out to find a deus ex machina to end the war, and they succeed in their search. There's no twist, no meaningful new complication. A technobabble explanation aside, there's little that couldn't have been predicted a dozen pages in.
Spock Must Die! isn't a bad book, and it's particularly interesting as a contrast to later Trek books. Blish's treatment of the characters is somewhat unique, and he certainly has no concern for maintaining the status quo. You could do worse than to spend a couple of hours reading this one.