Condition is VG+: tight and square with flat spine; spine has 2 lines; mild uniform age tanning (darker on inside of covers; what you expect for 40 years), very light overall wear. Stamps on first page & page 85 of each side - from the reserves of a defunct SF library (same source as the F&SF magazines I've sold in previous years.)

Ace Double (M-113) 1965 paperback: The Rithian Terror, bound with a collection Off Center - both sides by Damon Knight. This is the first book appearance for each.

The Rithian Terror - Cover and frontispiece by Jack Gaughan; 111 pages. Magazine version was titled "Double Meaning" in Startling Stories, January 1953.
The fate of the Earth Empire held in the balance - and Security Commissioner Spangler knew it was up to him to find the monster - the Rithian Terror, as some called it. Seven Rithians had landed on Earth, and six had been disposed of. Only one was loose. He explained to Pembum, a Colonial sent to help find the monster, that testing with the stereoscopic fluoroscope would surely flush it out. But Pembum pointed out that while the Rithi have no bones, nothing could prevent one from swallowing a skeleton...

(Bound with) Off Center - Cover and 5 small illustrations by Jack Gaughan; 141 pages.

What Rough Beast
The Second-Class Citizen
Be My Guest
God's Nose
Catch That Martian

These originally appeared in Magazine of F&SF, Fantastic Universe, Rogue, and Galaxy, from 1952 to 1964. Damon Knight is at his best at shorter lengths like these, and most of these are not available in any other Knight collection.