Time Traders

Blurb

The Time Traders is the first novel in The Time Traders series by Andre Norton. 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 series' premise, a confrontation between Western heroes, and the "Reds", and 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.