Barbara Hambly 
Crossroad [EPUB ebook] 

The crew of the
Nautilus, a battered Starship of mysterious origin, is beamed aboard the
Starship Enterprise™. The group claims they are freedom fighters from the future working to save the Federation from the Consilium — a group of corrupt power-seekers.
But when the
Nautilus crew members suddenly seize control of the
U.S.S. Enterprise™, and a Starship from the future arrives to arrest the renegades, Kirk must separate his true allies from those who wish to destroy the Federation.