Same cover art as the 1963, 1967 "short" Ace editions. This is the first full-height paperback from Ace. I like this version best - I think the smaller framing and purple background makes Schomburg's cover look a little less lurid.

The second of Andre Norton's "Hosteen Storm" books: Lord of Thunder. Hosteen and his beast team try to find the cause of native unrest on the planet Arzor.

Here's the flap-copy from the first hardcover, which gives a reasonable summary of the set-up:
"Why - at the height of the great dry season when neither man nor beast can long survive away from shelter and water - should all the native tribes on the planet Arzor be gathering in the sinister Peaks? To find the answer, Hosteen Storm, a Beast Master relocated here after Terra's destruction in a galactic war, is sent into the Peaks on a mission of utmost danger. With his great cat Surra and his African eagle Baku, a perfect scouting team, Storm and two companions discover a fantastic world underground and an uprising planned against the off-worlders. How can total disaster be diverted? That solution forms the dramatic climax.... "

For a better plot summary/set-up and some commentary on Norton, I rely on P. Schuyler Miller, and use excerpts from his review in Analog (February 1963):
"Hosteen Storm… is asked to undertake a mission into "the Blue" beyond the farthest explored ranges, where a lifeboat is thought to be down. But this is the very region to which all the Norbies - the horned natives of Arzor - have been summoned for a great powwow of civilized and savage tribes alike. It is a region of mystery and magic, where an ancient race once had a hidden stronghold and where remains of the Xik, the alien foes who destroyed Earth before they were themselves destroyed, may still be hiding.

Of course Hosteen Storm does go out into the Blue and does encounter mysteries of the Norbies and of the Ancient Ones… I keep raving over these books simply because I read them for the "plus" values that are in every one of them, quite apart from the maneuverings of the plot. I am not much concerned with what is going to happen next, but with how and why it happens and what the happening will do to fill in the incomparably rich tapestry the author is weaving. In this book we glimpse of the intricate society of the Norbies, and with Hosteen Storm begin to understand them as a people whose world is being usurped by mankind - as the America of Hosteen Storm's Indian ancestors was usurped centuries before by European invaders. We learn more about Arzor, completely strange and completely real, unlike Earth even in its similarities. We get more brief, vivid, paradoxial glimpses of the mighty lost civilization of the Ancient Ones.

Most important, to me at least, is the fact that as fully and richly as Andre Norton paints her pictures and weaves her tapestries of other worlds, she always leaves something for the reader to fill in… The very fact that all the loose ends are never neatly and explicitly tied up in the last chapter gives these stories a value far beyond the average." [P. Schuyler Miller]