Star Trek: The Yesterday Saga, Book One: Yesterday's Son

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!