This is the first Star Trek pulp novel with an original story arc—its predecessors were all adapted teleplays of the original 1960s series. The premise is that a transporter accident creates two Spocks, who are at first indistinguishable. Gradually, it becomes clear that one of them is a mirror image of the other, and therefore has an inverted Spock-soul, or something, which makes him want to destroy the Enterprise. Hence the title. It was a dumb but very fun read. I used to read Star Trek and Star Wars canon books all the time as a kid, so it was kind of a blast from the past. Please note that this is not the source of Evil Bearded Spock.