This is one of the best Star Trek novels I have read. That could be just because I like Spock. ;-) It is a sequel to "Yesterday's Son" which was Crispin's first published book, I think. I borrowed that one from the library and since they didn't have this sequel, I got it from PBS.
Part 2 of the 'Yesterday Saga', we see what has transpired with Zar, the hero from 'yesterday's son.' I did not like the characterization of Spock on this one, but that is a very personal opinion of mine. All and all, the book is well written. Sorry but if I give more details it will be spoilers, and I hate those.
One of the better Trek novels, centering around a malfunction in the Guardian of Forever which is wreaking havoc in the universe. Crispin's strength is her deep affection for and understanding of the characters as they age and change through the established TOS timeline.. She returns here to a pocket universe she created earlier, in which Spock's brief relationship with a woman caught in a doomed planet's ice age resulted in a son. The son is now an adult, and embattled in his own time, when Spock, Kirk, and McCoy travel through the Guardian to seek his help.
This is a sequel to Yesterday's Son. Again, a superior Trek novel.