Amazon.com Product Description (ISBN 0441272290, Mass Market Paperback)

Ace 8th printing features "reversed" Davis Meltzer cover (see backwards signature at bottom!) "Time-travel for archaeologists was a well-guarded secret; but when the remains of an interstellar spaceship are uncovered along with the usual fossils, the time agents must call on a modern Apache Indian, Travis Fox, to guide them in the ways of his ancestors in their trip to the past. But the explorers become trapped in space as well as time, on a mad journey on an automatic starship; not even Travis Fox can tell whether any of them will ever see Earth again."

Amazon.com Product Description (ISBN 0441272347, Paperback)

Three men undertake a perilous journey to investigate an alien spaceship wrecked during prehistoric times.

Amazon.com Product Description (ISBN 0671319523, Hardcover)

DRAFTED INTO THE ARMY OF TIME

Intelligence agents have uncovered something which seems beyond belief, but the evidence is incontrovertible: the USAs greatest adversary on the world stage is sending its agents back through time! And someone or something unknown to our history is presenting them with technologies -- and weapons -- far beyond our most advanced science. We have only one option: create time-transfer technology ourselves, find the opposition's ancient source...and take it dawn.

When small-time criminal Ross Murdock and Apache rancher Travis Fox stumble separately onto America's secret time travel project, Operation Retrograde, they are faced with a challenge greater than either could have imagined possible. Their mere presence means that they know too much to go free. But Murdock and Fox have a thirst for adventure, and Operation Retrograde offers that in spades.

Both men will become time agents, finding reserves of inner heroism they had never expected. Their journeys will take the battle to the enemy, from ancient Britain to prehistoric America, and finally to the farthest reaches of interstellar space...