The Last Strike — Peter Bostrom

His son is dead.
His ship is dead.
His career is dead.
But Spectre is very much alive. And with his plans of galactic domination laid bare, Captain James Mattis must gather what allies he can to stop him.
Admiral Pippa Spears and the HMS Caernarvon make a mysterious discovery in the wreckage of the USS Stennis, one that could change the outcome of the war. And in the uproar after the assassination of the American President she and Captain Mattis race across the galaxy after the suspected shooter, knowing full well the murder is another of Spectre's schemes to throw them off guard while he carries out his sinister plans.
Plans that Mattis and Spears are close to unraveling.
The cost of failure is unthinkable: a destroyed Earth and an unstoppable immortal foe.