Spectre

Spectre is a novel written by William Shatner with Judith and Garfield Reeves-Stevens and published by Pocket Books in hardcover, eBook, and eAudio formats. The novel continues the adventures of James T. Kirk after his resuscitation in The Return. It takes place before Dark Victory and after Avenger. A mirror universe novel, it takes place after DS9's "Shattered Mirror".

Summary
Kirk is kidnapped by a woman claiming to be Kathryn Janeway. All his training doesn't prepare him for who her commander is: none other than Spock. More disturbing is the fact that this Spock seems to have aged more than most humans, and then Kirk recognizes from his goatee that it's Spock (mirror). Kirk learns that Spock did more than consider Kirk's plea to reform the Empire--and it backfired and led to the Terran Empire being conquered by the Klingon-Cardassian Alliance. Kirk realizes that is why this Spock has aged so, and agrees to help the Terran and Vulcan rebels.

Information
early paperback edition, ISBN 0671020536 came out six months after hardcover edition and six months before mass-market paperback edition