Ace Double (M-105): a novel, Message from the Eocene, plus a collection, Three Worlds of Futurity - both sides by Margaret St. Clair (1964.) This is the first book appearance for each.

Three Worlds of Futurity - Cover and 5 small illustrations by Jack Gaughan; 142 pages.
The Everlasting Food
Idris's Pig - (Original title: "The Sacred Martian Pig")
The Rages - (Original title: "The Rations of Tantalus")
Roberta
The Island of the Hands

These originally appeared in Thrilling Wonder Stories, Startling Stories, Fantastic Universe, Galaxy,and Weird Tales - from 1949 to 1962. Most are not available in any other St. Clair collection.

(Bound with) Message from the Eocene - Cover and frontispiece by Jack Gaughan; 114 pages.
His name was Tharg, but he was not of any life form we know today. He lived so long ago that the planet Earth had not yet shaped itself. Lava seas roiled and churned, volcanoes spouted and grew, and heavy clouds hung in the hydrogen atmosphere, leaving the planet's surface dark and dangerous. On that world Tharg met his death, or something very much like it. He became a disembodied, totally nonphysical intelligence, cut off from all contact with the life he had known. He "slept" for hundreds of millions of years, unconnected with the world, unthinking, hardly existing. But then he began to awake - for there was new life on Earth, creatures called "human," and Tharg, knowing an ancient promise from the stars, had to tell them of it. But... how?