Key Out of Time by Andre Norton
Number of pages: 273
Time Agents Ross Murdock and Gordon Ashe, aided by a Polynesian girl and her team of telepathic dolphins, probe the mystery of the sea-planet men have named Hawaika. Its cities and civilizations have vanished but our agents are snatched back through a Time Gate and marooned in the midst of the struggle for power that must have destroyed the planet.
Download or read it online for free here: