The Time Traders

The Time Traders is a science fiction novel by American writer Andre Norton, the first in The Time Traders series. It was first published in 1958, and has been printed in several editions. It was updated by Norton in 2000 to account for real world changes. It is part of Norton's Forerunner universe.
The Time Traders introduces the premise: a confrontation between Western heroes and the "Reds", AKA the Soviets, plus the "Baldies", a mysterious alien race that has used time travel to alter Earth. This novel alternates among the present day, a trading tribal society in Britain, 2000 B.C., and a glacial outpost in the last ice age.

In her Time Trader novels Norton tacitly assumes that the physics of time travel differs so significantly from the physics of space travel, especially hyperdrive-propelled interstellar flight, that a civilization that discovers the technology of one simply will not discover the technology of the other. Earth's physicists have discovered the secret of time travel, but the engineers and scientists who built and use the time transporters have devised a clever way to obtain the secrets of space travel: if it is not possible to discover the secrets, we get them from someone who did.

Floyd C. Gale wrote that "Traders gets Miss Norton back solidly and admirably on her track of excellence", and was one of her "topnotchers".
In the Saturday Review for 1958 Nov 01, the reviewer, identified by the initials A. O'B. M., wrote:
This exciting story is set in the last quarter of this century, but the action takes place in several levels of time and periods of history and prehistory. A young convicted criminal becomes a volunteer in a government program which trains men to do research in these various eras. Object: the discovery of scientific secrets lost before the dawn of history. The hero is pictured as a kind of commando personality, a nonconformist in civilized society but the ideal man in primitive times.
In Kirkus Reviews for 1958 Oct 01[5] an unidentified reviewer wrote:
At the end of this century Ross Murdock is given the choice between prison and a dangerous role in a secret mission. Accepting the latter, but determined to escape at the first opportunity, the intelligent young man finds himself involved in a project which demands that he be projected back to various periods in history. For the Americans, aware that the Russians, somewhere in time, have learned the secret of space travel, must for the sake of national safety, obtain the same secret. Hurled back into the earliest ages of man, Ross' volatile intelligence is, for the first time, stimulated as he risks death, posing as a member of prehistoric worlds. By the time the Americans gain control of the secret, Ross is rehabilitated and is a willing participant in the benevolent army of the future. An interesting idea, well handled by Andre Norton, science fiction expert, who projects his (sic) reader deftly both backwards and forwards in time and injects his (sic) narrative with considerable and interesting historical information. (wikipedia.org)