Key Out of Time

Key Out of Time is a science fiction novel by American writer Andre Norton, set on the world, Hawaika, that appears to be a tropical paradise.
It is the fourth in Norton's series starts in The Time Traders, part of her Forerunner universe, and continues the series' premise: A hostile encounter between Western heroes, and the Russian Communists, and the Baldies - a mysterious alien race that used time travel to alter Earth. In the previous novel, The Defiant Agents, events are treated as a Time Agent failure - but re-read that novel for a different interpretation.

Kirkus Reviews, strongly supportive after following the series for years, writes:
Again, Andre Norton, one of the greats among writers of teenage science fiction (Galactic Derelict, 1959, p. 658, J-316, for one), has employed [her] boundless imagination ... Owing to the author's exceptional mastery of detail, and astute control of plot, Key Out of Time stands as a novel which should more than satisfy young science fiction fanciers and fanatics. (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)