Sarek: Star Trek Pocket Books, Book 69 by A. C. Crispin
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Summary
Spock's mother, Amanda Grayson, is dying, and Spock returns to the planet Vulcan where he and Sarek enjoy a rare moment of rapprochement. But just as his wife's illness grows worse, duty calls Sarek away, once again sowing the seeds of conflict between father and son. Yet soon Sarek and Spock must put aside their differences and work together to foil a far-reaching plot to destroy the Federation, a plot that Sarek has seen in the making for nearly his entire career.