The Ruins of Anthalas — Richard Fox

An ancient holocaust holds the key to humanity's survival.
Only a sliver of mankind survived the Xaros invasion. With Earth's defenses in ashes, nothing can stop the aliens' inevitable return. Hope arrives from a cryptic message from a long-lost alien race, promising the means to rebuild the shattered space fleet. Captain Valdar takes one of the last strike carriers, the Breitenfeld, and his shell-shocked crew on a desperate mission to the dead world.
Unfortunately, humans aren't the only power that received the ancient message...
THE RUINS OF ANTHALAS is the second book in THE EMBER WAR SAGA, a military sci-fi space opera. If you like David Weber and John Ringo with a touch of Isaac Asimov, then you'll love this fast-paced and captivating adventure where humanity survives at the edge of a knife.