Ace (F-320) 1965 paperback. Cover by John Schoenherr, frontispiece by Jack Gaughan, 149 pages, 40 cent cover price. Condition is VG+ to near-Fine: tight and almost square with flat spine, (spine has some edgewear, but nothing I can call a wrinkle or crease); age tannning is mild and uniform; very light overall wear. Stamp on first page, top page-edges & page 85 - from the reserves of a defunct SF library (same source as the IF magazines I sold years ago.)

John Brunner (1934-1995) was well-known for writing (a lot of) "literate space operas," many of which appeared in Ace-Doubles. During the Sixties, his writing style matured, and he started getting Hugo nominations, culminating with the Hugo winner in 1969: Stand On Zanzibar. Brunner wrote 4 novels and a handful of stories as "Keith Woodcott" - this is the only one (as by Woodcott) that didn't appear in Ace Double format. To my knowledge, none of the "Woodcott" titles have been reprinted elsewhere, making them some of the hardest titles to find by Brunner.

The Martian Sphinx: this appears to the first publication in any form. It has been decades since I read my copy, so here's the first page blurb:
WAS IT A MESSAGE FROM MARS OR AN INTERPLANETARY TRAP?
When he came in and peeled off his oxygen mask, his face was transfigured, and the whites of his eyes stood out perfectly round as he stared at the memory and gave his report in a threadlike whisper:
"I drove close and there she was. A woman. At first, I thought she must be some kind of an illusion - I mean, a woman out there, wandering around Mars in some kind of flimsy, close-fitting clothes... But she proved she wasn't a phantom. She came and gave a good hard rap on the door of the crawler."
"Listen! They're like us. They come from the Greater Magellanic Cloud, and something from another galaxy has moved in and is destroying them. It's coming towards our galaxy now - something monstrous and greedy and hostile, so they wanted to warn us, and she volunteered to come. We've got to go back and warn Earth at once!"
He put his face in his hands and began to sob.