Sector Eight (Perimeter Defense
Atamanov, Michael Perimeter Defense is a four-book series by Michael Atamanov marrying LitRPG with space sci-fi. In it, humanity has been roaming deep space for millennia, aggressively expanding their sphere of influence - until it runs into a strong and merciless enemy. Now the Terrans keep losing frontier battles and are forced to retreat, losing planet after planet. All attempts to stop the alien invasion or to at least try to communicate with them fail.
Sti...