The Last Fleet — Eric S. Brown

Taskforce Hope, the sole remaining human battle fleet, carries on the war against the insectoid race that wiped out the Republic and brought an end to human civilization two years earlier. It's a war the Taskforce can't hope to win. Outnumbered and outgunned, Admiral Powell still refuses to surrender, using hit and run tactics to make the Bugs pay for what they've done as best he can.