Andre Norton 
The Time Traders 

If it is possible to conquer space, then perhaps it is also possible to conquer time? At least that is the theory American scientists are exploring in an effort to explain the new sources of knowledge the Russians possess. Perhaps Russian scientists discovered how to transport themselves back in time in order to learn long-forgotten secrets of the past. Transferred by time machine to the remote Baltic region where the Russian post was located, Ross and his partner Ashe are swept into a fantastic action-filled adventure involving Russians, pre-historic men, and the aliens of a lost galactic civilization that demand every ounce of courage the Americans possess.