A.C. Crispin 
Yesterday’s Son [EPUB ebook] 

The Romulans attack the planet Gateway, where Federation scientists are studying the Guardian of Forever — the mysterious portal to the past. The Starship Enterprise must protect the Guardian — or destroy it. But Spock has already used the portal to journey to the past. On the planet Sarpedion, 5, 000 years ago, Spock knew a beautiful, primitive woman. Now he has gone back to meet his son!