Key Out of Time

Blurb

Key Out of Time is the fourth novel in The Time Traders series by Andre Norton. It was first published in 1963, and as of 2012, had been reprinted in 17 editions with cover changes, as well as twice in a combined edition with The Defiant Agents. It is part of Norton's Forerunner universe.
Key Out of Time continues the series’ premise, an encounter between Western heroes, and the Russian Communists, and the Baldies, a mysterious alien race that used time travel to alter Earth. Events in the previous novel, The Defiant Agents, are treated as a Time Agent failure — but read that novel for a different interpretation. The setting of this novel is a world, Hawaika, appearing to be a tropical paradise.
Kirkus Reviews, strongly supportive after following the series for years, writes, “Again, Andre Norton, one of the greats among writers of teenage science fiction, 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.”