omnibus of: DEFIANT AGENTS (1962) + KEY OUT OF TIME (1963) / P. Schuyler Miller once said this about Norton's "Time Trader" series: "...the richness which Andre Norton lavishes on her portraits of the wonder-worlds of the Universe, the subtle warmth of the empathic relations she portrays between men and mutated animals... and above all the mysteries she suggests and half-reveals but does not explain away with rationalizations - these, in the words of another book, are stories, O my brothers!" I agree, and will cheerfully recommend any of the first 4 in this series (...but not the co-authored later entries.)

Key Out of Time - a 1963 novel by Andre Norton - 4th in her "Time Traders" series. Here's the flap-copy from the first hardcover, which gives a reasonable summary or set-up:

Sent from Terra to probe the past of the deserted planet Hawaika, Time Agents Ross Murdock and Gordon Ashe are startled when murky underwater pictures of ancient races locked in a fierce battle for supremacy suddenly appear on their time-probe. Despite prophetic warnings of Karara, a Polynesian girl, the Agents decide to invade the past and uncover the secrets of this barren, lotus world.

Aided by Karara and the tclepathic powers of two dolphins, Murdock and Ashe set up an underwater time gate and are, by mischance, separately catapulted back ten thousand years into the midst of the competing Wreckers, Rovers, and Foanna - ancient peoples of Hawaika. Thus begins Murdock's perilous adventures among the Rovers, a highly mechanized maritime race, and the Foanna, enigmatic remnants of an occult civilization, who rule the rock-bound citadel above an unknown sea where Ashe is held. But the sudden appearance of the Baldies, blue-suited, hairless men from outer space who dominate by reading men's thoughts, poses a new threat to the Agents and the Hawaikans, too. Unwillingly drawn into a desperate attempt to save the planet from destruction, the Agents and their allies, including the dolphins, rely upon powers - natural and supernatural - in a fantastic other-world battle.