Time for Yesterday

Blurb

Time For Yesterday is a novel by A. C. Crispin set in the fictional Star Trek Universe. It is a sequel to Crispin's earlier novel, Yesterday's Son, and describes a second encounter between the crew of the USS Enterprise and Spock's son, Zar.
The two books followed the original series episode "All Our Yesterdays", and Time For Yesterday is subtitled The Yesterday Saga, Book 2.