Andre Norton 
Time Traders 

The first novel in the classic series, in which wars are no longer fought on land, in the air, or on the sea—but throughout time and space itself . . . The never-ending tension between Russia and the United States has been escalated by a stunning new variable: time travel. While the Americans can roughly utilize the technology, the Russians have learned the secret of space travel—and that makes the US vulnerable. What they need is someone expendable to send through time to obtain the same secret.   Ross Murdock doesn’t consider himself expendable. He’s just a smart guy who made some bad choices that led him to a life of crime. But when he’s given a choice between medical “rehabilitation” and joining the mysterious Operation Retrograde, he decides to go along with the military option and hope for a chance to get away.   And he is going to get away—farther away than he ever imagined . . .The Time Traders is the first book in the Time Traders Series, but you may enjoy reading the series in any order.