The Defiant Agents

The Defiant Agents is a science fiction novel by American writer Andre Norton, the third book in her Time Traders series. It was first published in 1962, and as of 2012, had been reprinted in ten editions with cover changes, as well as twice in a combined edition with Key Out of Time. It is part of Norton's Forerunner universe.
The Defiant Agents continues the series' premise, an encounter between Western heroes, and the Russian Communists, and a mysterious alien race that has used time travel to alter Earth. Most interaction in this novel is in an American Indian-like setting.
Kirkus Reviews comments, "Andre Norton, a polished writer of science fiction, weaves [her] outlandish fare in patterns intricate enough to attract the most devoted addict. Do not try to contrive a consistent political allegory out of [her] flight into fantasy. This is pure science fiction ..." (wikipedia.org)
About the author:
Andre Alice Norton (born Alice Mary Norton, February 17, 1912 - March 17, 2005) was an American writer of science fiction and fantasy, who also wrote works of historical and contemporary fiction. She wrote primarily under the pen name Andre Norton, but also under Andrew North and Allen Weston. She was the first woman to be Gandalf Grand Master of Fantasy, to be SFWA Grand Master, and to be inducted by the Science Fiction and Fantasy Hall of Fame.

Often called the Grande Dame of Science Fiction and Fantasy by biographers such as J. M. Cornwell, and organizations such as Science Fiction and Fantasy Writers of America, Publishers Weekly, and Time, Andre Norton wrote novels for more than 70 years. She had a profound influence on the entire genre, having more than 300 published titles read by at least four generations of science fiction and fantasy readers and writers. Notable authors who cite her influence include Greg Bear, Lois McMaster Bujold, C. J. Cherryh, Cecilia Dart-Thornton, Tanya Huff, Mercedes Lackey, Charles de Lint, Joan D. Vinge, David Weber, K. D. Wentworth, and Catherine Asaro.
On February 20, 2005, the Science Fiction and Fantasy Writers of America, which had honored Norton with its Grand Master Award in 1984, announced the creation of the Andre Norton Award, to be given each year for an outstanding work of fantasy or science fiction for the young adult literature market, beginning with 2005 publications. While the Norton Award is not a Nebula Award, it is voted on by SFWA members on the Nebula ballot and shares some procedures with the Nebula Awards. Nominally for a young adult book, actually the eligible class is middle grade and young adult novels. This added a category for genre fiction to be recognized and supported for young readers. Unlike Nebulas, there is a jury whose function is to expand the ballot beyond the six books with most nominations by members.
Norton received the Inkpot Award in 1989. (wikipedia.org)