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Kirk being the exception in everything for Spock in this book is absolutely crushing.
Spoilers for The Entropy Effect:
Early on they establish that Spock maintains a strict distance from his other crewmates, and other beings in general as being a touch telepath means he has to maintain shields against their emotions and thoughts. Braithewaite shaking his hand is such a faux pas because of this, Vulcans are known to be touch telepaths and he should know better! Kirk and Spock are both irked by the gesture to the point that Kirk notices Spock is giving Braithewaite a hard time (politely).
Kirk is shot on the bridge and without a second thought Spock is there, already trying to staunch the bleeding, holding Kirk in his arms (using his first name a lot too…) and drops literally every shield he has. They’re so entwined that even dying Kirk recognizes that Spock’s going to follow him into death.
After Kirk’s death, Spock is in charge of the Enterprise as it was Kirk’s last request that he take care of his ship, but Spock’s already planning on transferring off as soon as possible- it doesn’t occur to him that he could stay there under a different captain, which would be the most logical course of action. Jim’s death ends that part of Spock’s life that takes place on the Enterprise.