Novel about defeated humanity staging a comeback by discovering an ancient starship?

Question

A friend of a friend is looking for a paperback SF novel he vaguely remembers, possibly from an Ace Double. I was one of those asked, but didn’t recognize it. Here's what he recalls:

Mankind goes out to the stars, but runs into an established empire, gets into a war, and Earth is wiped out. All that's left are the Space Navy ship crews and members on bases. They live in what amounts to refugee ghettos on various planets.

Then one of the crew members finds an ancient ship, one predating the empire that defeated them. The builders might be the Kree(?) or Klee(?). Its capabilities far exceed those of the ships used by the aliens who conquered them.

So they pick up all the humans they can find to crew the ship, and proceed exact revenge.

Answer 1

In a comment Moshe suggests that the book could be Recall Not Earth by C. C. MacApp. I have the book (though I don't recall ever having read it) and I can confirm this is the book described in the question.

The summary from Goodreads is:

All life on Earth had been destroyed in a quick and easy kill by the great and ruthless intergalactic Vulmot Empire. Only an elite group of spacemen survived the holocaust. The last earthlings, they were scattered wanderers through the galaxies, hiring out their fighting skills to whatever power needed them, yearly growing more bitter, dissolute, desparing.

By now the pitiful remnants of the Space Corps lacked even the will to fight - until a tantalizing promise from an enslaved race made them assemble one last time, for a mission only men with nothing to lose would attempt, for a final battle only desperate courage could win...

which matches the basic scenario described in the question, and the derelict starship was indeed built by a vanished race called the Klee. The location of the ship is revealed to the surviving Earthmen by a member of a race called the Chelki as part of a bargain with them:

The Omniarch returned the photograph to the envelope he'd taken them from, put the envelope in his folder, and laid the folder on the small table. "You have perhaps seen a few Klee artifacts, and heard various legends."

John shrugged. "Who hasn't? Bits of statuary, cast from metals no one can duplicate and that have lasted twenty or thirty thousand years without corroding. Smashed instruments that baffle the best of scientists. Kitchen implements, jewelry…"

"I," the Omniarch said, "know where I can get you a Klee ship, intact, powered, and operative. Not primarily a warship, though she has bays for weapons to be mounted. A very large ship. And I will help you solve the controls and instruments. I have made a considerable study of Klee technology and have made a few inroads."

The ship is taken by the Earthmen and (re)named Bertha.

The aliens that destroyed the Earth are called the Vulmoti. At the end of the book there is a confrontation with the Vulmoti forces during which:

The null space drive on the Bertha malfunctions and jumps ten thousand years into the future. The Earthmen find that during this time the Vulmoti empire collapsed and is now forgotten, and that the Earth is now inhabitable again.

Answer 2

After Doomsday by Poul Anderson could be a good match.

Published in 1962, the novel opens with the returning crew of an American starship to Earth, only to find that Earth is no more: it has been destroyed. There are aliens involved and I seem to remember the theft of a powerful alien starship by the surviving humans.

I don't remember much more about this novel that I've read good thirty years ago but the Wikipedia has a detailed summary: https://en.wikipedia.org/wiki/After_Doomsday

Answer 3

Could this be Brandon Sanderson's book Skyward?

Spensa is a 17-year-old girl who is part of a group of shipwrecked humans living on a ruined world called Detritus, under constant attack from mysterious aliens called the Krell. Spensa dreams of following in the footsteps of her deceased father, a fighter pilot of the Defiant Defense Force (DDF). However she is barred from any chance at becoming one because her father abandoned his flight in the infamous Battle of Alta, which ended in his own wingmates shooting him down.

....

Spensa is barred from using academy facilities and quarters, so she occupies a nearby cave, where she discovers an advanced crashed ship with an artificial intelligence computer. Spensa, with the help of a cautious Rodge, begins to work on repairing the ship. In the meantime, Spensa and her cadet flight crew continue to learn and practice new techniques under the guidance of Cobb. Spensa begins to bond with her classmates.

....

On graduation day, Ironsides mobilizes all her pilots to battle the Krell for a valuable piece of fallen space salvage. While the pilots are distracted, a Krell bomber launches an attack from the opposite direction. Spensa uses a damaged fighter to assault the bomber, but is shot down again. She crashes, but Cobb brings M-Bot to her, and she uses its advanced capabilities to destroy the bomber and save the DDF. She travels with M-Bot into space, where her Defect allows her to hear and intercept Krell communications. She returns to the planet to report that the Krell are actually a coalition of aliens intent on keeping humans trapped on Detritus, though they are now actively trying to exterminate humanity.