Norton, Andre: The Time Traders, v1 27 Jun 2008
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 down.
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...