Jack Campbell 
Invincible [EPUB ebook] 

The war-weary Alliance First Fleet, commanded by Admiral John "Black Jack" Geary, is scores of lightyears from human-controlled space. After narrowly escaping the deadly enigma race they were sent to evaluate, the fleet is facing a second, even more hostile, alien species in an unknown star system. Geary is determined to make it home before danger can strike humanity again. To fight his way out of the alien trap, all he has to do is hold the fleet together, despite everything that threatens to break it apart.