Amazon review:

Message from the Eocene begins several billion years in the earth's past. Technically speaking, this would have been the Paleoproterozoic Era.

I think the word he was looking for was "actually."

A humanoid creature only known as Tharg is trying to escape the lighting bolts being hurled at him by an unknown source. He's on the surface of the primitive planet Earth on an important mission for the floating city, Synon. He's a member of a specialized group known as "divers" who travel to the dangerous planetary surface in search of valuable minerals needed by the city. Tharg has been entrusted with a container carrying a mysterious book for delivery to the sister city of Gwynor. He's not sure who is tossing the lighting bolts at him, but suspects they are from their enemies, the Veidimi.

When he's captured it turns out the source of the lighting was the Vaeaa, the "half-mythical overlords" who secretly control the third planet from the sun. By now, Tharg has had the chance to open the case holding the book and

Whatever. St. Clair also wrote as Idris Seabright and Wilton Hazzard. Her bio states:

In her rare autobiographical writings, St. Clair revealed few details of her personal life, but interviews with some who knew her indicate that she and her husband were well-traveled (including some visits to nudist colonies), were childless by choice, and in 1966 were initiated into Wicca by Raymond Buckland, taking the Craft names Froniga and Weyland.

As much as I love sci-fi, there is often something . . . peculiar about the authors.